How to create a Drop Down menu for a script file?

You can use an enum....

 public class MainClass : MonoBehaviour
 {
      public enum Type{ One, Two, Three}
      public Type type;
 
      void Start()
      {
           //Example usage
           switch(type)
           {
                case Type.One:
                     Debug.Log("One");
                break;
 
                case Type.Two:
                     Debug.Log("Two");
                break;
 
                case Type.Three:
                     Debug.Log("Three");
                break;
           }
      }
 }