How to spawn different object from Prefabs folder

Hello friends, im beginner coder. I have problem with spawning randomly 6 prefabs . These prefabs have specific position at Y. How can adjust my code any idea ?

 public class Spawner : MonoBehaviour {
 
     Vector3 spawnerPosScreen;
     float timer;
 
     public GameObject obstacle_RED_DOWN;              // Down means you need crouch
     public GameObject obstacle_RED_DIV;                // DIV means you need to jump half
     public GameObject obstacle_RED_UP;             // UP means you need to Jump
     public GameObject obstacle_BLUE_DOWN;     // Blue and Red two type obstacles you need jump etc
     public GameObject obstacle_BLUE_DIV;
     public GameObject obstacle_BLUE_UP;
 
     public float delayTimer = 0.5f;
 
 
     void Start ()
     {
 
         timer = delayTimer;
     }
     
     
     void Update ()
     {
 
 // This part attach the spawner end of viewpoint

         Vector2 spawnerPosScreen = Camera.main.WorldToScreenPoint(transform.position);
         if (spawnerPosScreen.x < Screen.width)
         {
             transform.position =
                 Camera.main.ScreenToWorldPoint(
                     new Vector3(Screen.width,
                                 spawnerPosScreen.y,
                                 transform.position.z - Camera.main.transform.position.z));
         }
         else if (spawnerPosScreen.x > 0.0f)
         {
             transform.position =
                 Camera.main.ScreenToWorldPoint(
                     new Vector3(0.0f,
                                 spawnerPosScreen.y,
                                 transform.position.z - Camera.main.transform.position.z));
 
         }
             timer -= Time.deltaTime;
 
         if (timer <= 0)
         {
 
             GameObject obstacle = (GameObject)Instantiate(obstacle_RED_DOWN, transform.position, transform.rotation);
             timer = delayTimer;
             Destroy(obstacle, 3F);
         }   
     }
 }