Implementing Level ID

Hello, everyone

I am working on a script for a school assignment where I have to implement a level ID or reference for use in case later levels are implemented to easily transition between them. Does anyone have any idea how to do this? I placed the exact instructions and the core game script down below. Plz let me know how I can go about doing this.

using UnityEngine; using System.Collections;

public class CoreGame : MonoBehaviour {

 // Reference to the pause menu
 [SerializeField] GameObject pauseMenu;
 // Variable of the handle paused function that checks if it were true. 
 private bool paused = false;
 // This is for initialization
 void Start () {
     // Start a screen fade up
     ScreenFader.CreateFade("ScreenFadeUp", FadeUpCompleted);
     // Hide the pause menu initially
     HandlePause (false);
     }
 // Helper to attach a game object as a child to the core game
 static public void MakeChild (GameObject _child) {
     // Find the "instance" of the core game object.
     GameObject coreGameObj = GameObject.Find("CoreGame");
     // Assign the child's transform's parent to the 
     // recently found core game's transform.
     _child.transform.parent = coreGameObj.transform;
 }
 // Invoked when the fade has completed.
 void FadeUpCompleted () {
             Debug.Log ("Fade UP has completed!");
 }
 // Update is called once per frame
 void Update () {
     CheckInput();
 }
 // Polls user input
 void CheckInput () {
             // User has pressed pause?
             if (Input.GetKeyUp (KeyCode.P)) {
                 HandlePause(!paused);
             }
     }
 // Handles the actions of the pause event
 public void HandlePause(bool _pause) {
             // Assign new pause status
             paused = _pause;
             // Send a message to all children
             BroadcastMessage ("SetPaused", paused);

             // Halt or start the progression of time
             if (paused)
                     Time.timeScale = 0;
             else 
                     Time.timeScale = 1;
             // Change the menu's visibility 
             pauseMenu.SetActive (paused);
     }

}