Add a dropdown menu option on the unity inspector from a script

DropDownMenuOnScript.cs

using UnityEngine;
public class DropDownMenuOnScript : MonoBehaviour
{    
    public enum Option { one, two, three, four, five }; // declare your dropdown list enumerator class with the options you want
    public Option option = Option.one; // instantiate your enumerator (it works both with and without initialization)
   
    ...
    // your methods
    ...
}