How to add score only one time after pressed a button

Declare a bool such as hasPressedRightAnswer that is initially false. Inside AddScore method, first check the bool to be false, then add score if it is.Next line inside method, you should change the bool value to true. Also you should change it back to false exactly when the next question appears. Alternatively, in your script, you can simply uncheck "Interactable" property of the button when the the right answer is pressed. then check it when new question appears.

     public class YourClassName
     {
         private bool hasPressedrightAnswer = false;
         private int score;
         private void AddScore()
         {
             if (hasPressedrightAnswer == true)
             {
                 return;
             }
             ++score;
             hasPressedrightAnswer = true;
         }
         // And when the next question appears, do this in some method:
         hasPressedrightAnswer = true;
     
     
         // Alternatively
         [SerializeField] Button btn;
         private void AddScore()
         {
             ++score;
             btn.interactable = false;
         }
         // And when the next question appears, do this in some method:
         btn.interactable = true;
     }