Why a public varibale is not accsible inside Update() ?

 using UnityEngine;
 using System.Collections;
 
 public class objectControl : MonoBehaviour {
     
     public char slected_item = 'F';
     public int selected_model = 0;
     public GameObject f,o,d;
     
     // Use this for initialization
     void Start () {
         f = GameObject.Find("f");
         o = GameObject.Find("o");
         d = GameObject.Find("d");
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetKey(KeyCode.F)){
             selected_item = 'F';
             doSelectedAnimation();
         }
     }
 
     void doSelectedAnimation(){
         iTween.MoveTo(f,iTween.Hash("y",-3,"time",4));
     }

}

I get the following error

Assets/Scripts/objectControl.cs(20,25): error CS0103: The name `selected_item' does not exist in the current context