How to make questions & answers random in my script C#?

The easiest way to implement this is to create instances of a question class (preferrably not in a scene but in the project folder) which hold the question and the possible answers and let them handle randomizing the button positions.

 using UnityEngine;
 using System.Collections.Generic;
 
 // A Question
 public class MyQuestion : ScriptableObject {
 
     // the question
     public string question;
 
     // the different answers (answer[0] is right answer by default)
     public string[] answers;
 
     // the order, in which answers are displayed
     private int[] m_order;
 
     //This does not work outside the Editor
 #if UNITY_EDITOR
     // Create Questions from Folder Context Menu
     [UnityEditor.MenuItem("Assets/Create/Question")]
     public static void CreateQuestion()
     {
         string resourcePath = "Assets/Resources/Questions";
 
         // makes sure you have a directory to save your question to.
         if (!System.IO.Directory.Exists(Application.dataPath + "/Resources"))
         {
             UnityEditor.AssetDatabase.CreateFolder("Assets", "Resources");
         }
         if (!System.IO.Directory.Exists(Application.dataPath + "/Resources/Questions"))
         {
             UnityEditor.AssetDatabase.CreateFolder("Assets/Resources", "Questions");
         }
 
         // create question
         MyQuestion myQuestion = CreateInstance<MyQuestion>();
         
         // name question
         int questions = UnityEditor.AssetDatabase.FindAssets("", new string[1]{resourcePath}).Length;
         myQuestion.name = "Question_" + questions;
 
         // save question to project folder
         UnityEditor.AssetDatabase.CreateAsset(myQuestion, resourcePath + "/" + myQuestion.name + ".asset");
     }
 #endif
 
     public void OnEnable()
     {
         if (question == null)
         {
             question = "";
         }
         if (answers == null)
         {
             answers = new string[4] { "", "", "", "" };
         }
         // Prevents exceptions in OnGUI if you forget to call RandomizeAnswers() 
         m_order = new int[answers.Length];
         for (int i = 0; i < m_order.Length; ++i)
         {
             m_order[i] = i;
         }
     }
 
     // Randomizes the order in which answers are presented
     public void RandomizeAnswers()
     {
         System.Random random = new System.Random();
         List<int> order = new List<int>();
         
         // get indices
         for (int i = 0; i < answers.Length; ++i)
         {
             order.Add(i);
         }
 
         m_order = new int[answers.Length];
         int index = 0;
 
         // shuffle indices
         while (order.Count > 0)
         {
             int index2 = random.Next(order.Count);
             m_order[index] = order[index2];
             order.RemoveAt(index2);
             ++index;
         }
     }
 
     /* Your Question display function
      * returns true (right answer), false (wrong answer) or null (nothing chosen) */
     public bool? ShowQuestion(Rect _area)
     {
         // bool that can be true, false or null. ? makes any struct nullable
         bool? result = null;
 
         // just a simple layout
         GUILayout.BeginArea(_area);
 
         // display question
         GUILayout.Label(question);
 
         for (int i = 0; i < answers.Length; ++i)
         {
             // display buttons in random order
             if (GUILayout.Button(answers[m_order[i]]))
             {
                 if (m_order[i] == 0)    // right choice
                 {
                     result = true;
                 }
                 else                    // wrong choice
                 {
                     result = false;
                 }
             }
         }
         GUILayout.EndArea();
 
         return result;
     }
 
 }

(You can save instances of classes that inherit from ScriptableObject as assets in in your project)

Now to randomize the questions themselves, one way to go is use a central question pool that holds all questions and ask for new question every time the question trigger is entered.

 using UnityEngine;
 using System.Collections;
 
 // Singleton for managing the question pool
 public class MyQuestionPool {
 
     // static instance
     private static MyQuestionPool m_questionPool = null;
 
     // all questions
     private MyQuestion[] m_questions = null;
 
     private System.Random m_random = null;
 
 
     private MyQuestionPool()
     {
         m_random = new System.Random();
         InitQuestions();
     }
 
     // Loads questions
     private void InitQuestions()
     {
         /* Resources.Load looks inside the Assets/Resources-folder
          * full path is Assets/Resources/Questions */
         m_questions = Resources.LoadAll<MyQuestion>("Questions");
     }
 
     // Gets the question pool
     public static MyQuestionPool Instance
     {
         get
         {
             if (m_questionPool == null)
             {
                 m_questionPool = new MyQuestionPool();
             }
             return m_questionPool;
         }
     }
 
     // Gets a random question
     public MyQuestion GetQuestion()
     {
         if (m_questions != null && m_questions.Length > 0)
         {
             int index = m_random.Next(m_questions.Length);
             m_questions[index].RandomizeAnswers();
             return m_questions[index];
         }
 
         return null;
     }
 }
 

At last, the trigger:

 using UnityEngine;
 using System.Collections;
 
 // A question trigger
 public class MyQuestionTrigger : MonoBehaviour {
 
     // current question
     private MyQuestion m_question = null;
 
     public void OnGUI()
     {
         // Display Question
         if (m_question)
         {
             bool? result = m_question.ShowQuestion(new Rect(Screen.width / 2 - 200, Screen.height / 2 - 200, 400, 400));
 
             if (result == null)
             {
                 // no answer, wait...
                 return;
             }
             else if (result == true)
             {
                 // right answer given
             }
             else
             {
                 // wrong answer given
             }
 
             // discard question
             m_question = null;
         }
     }
 
     public void OnTriggerEnter(Collider _col)
     {
         // Get Question
         m_question = MyQuestionPool.Instance.GetQuestion();
     }
 }

With this you can add questions until you run out of RAM, Array-Indices or ideas for new questions. You can edit them in your inspector. Just do not delete questions or it will screw with the way new questions are named.