Word Spelling game ,how to add the letter to the word when click the button?

Hi all , I'm making a word spelling game , every words will have two blank. And there have four buttons that you can choose. When click the button will add the correct or wrong letter to the word. How to add a letter at a time and add a letter to the word ?

Like this:

Letter.CS:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 
 public class Letter : MonoBehaviour
 {
     public char character;
     public RectTransform rectTransform;
     public Text text;
     public int index;
 
 
     public Letter Add(char c)
     {
         character = c;
         text.text = c.ToString();
         gameObject.SetActive(true);
         return this;
     }
 
 
 }

SpellGame.CS:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class SpellGame: MonoBehaviour
 {
     
     private DataCon dataCon;
     
     public char[] arr;
 
     public float space;
     public float lerpSpeed=5;
     public Transform parent;
     public Letter prefab;
     List<Letter> charObjects = new List<Letter>();
 
     public int currentWord;
 
     void Start()
     {
        
         dataCon = FindObjectOfType<DataCon>();
         ShowTo(currentWord);
     }
 
     public void ShowTo(int index)
     {
         arr =dataCon.allQuiz[0].questions[Random.Range(0,dataCon.allQuiz[0].questions.Length)].queText.ToCharArray();
         for (int i = 0; i < 2; i++)
         {
             arr[Random.Range(0,arr.Length)] = '_';
         }
 
         foreach (char c in arr)
         {
             Letter clone = Instantiate(prefab.gameObject).GetComponent<Letter>();
             clone.transform.SetParent(parent);
 
             charObjects.Add(clone.Add(c));
         }
         currentWord = index;
        
     }
 
    
 
 
     void RepositionObject()
     {
         if (charObjects.Count == 0)
         {
             return;
         }
 
         float center = (charObjects.Count - 0.9f) / 2;
         for (int i = 0; i < charObjects.Count; i++)
         {
             charObjects[i].rectTransform.anchoredPosition = Vector2.Lerp(charObjects[i].rectTransform.anchoredPosition, new Vector2((i - center) * space, 0), lerpSpeed * Time.deltaTime);
             charObjects[i].index = i;
         }
     }
 
    
     void Update()
     {
         RepositionObject();
        
     }
 
 
 }

AddButton.CS:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class AddButton: MonoBehaviour
 {
     public Text text;
 
     private WordGame wordGame;
     private Letter letter;
 
 
     private void Start()
     {
         wordGame = FindObjectOfType<WordGame>();
         letter = FindObjectOfType<Letter>();
     }
 
 
     public void ClickButton()
     {
 
         char[] c = text.text.ToCharArray();
         
         for (int i = 0; i < wordGame.arr.Length; i++)
         {
             if (wordGame.arr[i]=='_')
             {
                 wordGame.arr[i] = c[0];
                 letter.character = c[0];
                
             }
         }
 
 
     }
 
 
 }