You don't even bother text after the [a] at first place in this case
Your question is something like
anwser only a,b,c,d for following question
a. b. c. d....
the model output possibility of next character
a: 0.8 b: 0.7 c: 0.3 f: 0.2 d: 0.1
If the list contains option you did not provide.
The model is confused anyway, it don't matter if you use grammer to filter out the bad option or not, the answer is screwed already.
Your question is something like
anwser only a,b,c,d for following question a. b. c. d....
the model output possibility of next character a: 0.8 b: 0.7 c: 0.3 f: 0.2 d: 0.1
If the list contains option you did not provide. The model is confused anyway, it don't matter if you use grammer to filter out the bad option or not, the answer is screwed already.