The correct answer will be known before the player chooses, so the green object should already have an instance variable where value 'correct' or 'incorrect' is assigned. When the player chooses an answer, you can make the green objects where value=incorrect invisible. So what i'm saying is you pick by a variable value, not by UID.
However when I look at your post above, you say that all the green answer objects go invisible anyway, so is there a need to pick one ? Unless you meant that all of them going invisible is just the 'current functionality' and a bug, in which case you can do the above.