Which is better

A is overly formal. B would be far more common in conversations between brothers.

Thank you, AlpheccaStars. Is it possible to use if + would as someone says that you can never use "would" or "could" in IF clause?

Desafinado if + would as someone says that you can never use "would" or "could" in IF clause?

That someone does not know English very well.

I would come if I could.
If he would only study harder, he could win a scholarship.

Thank you. But I still do not know the reason why the answer is B. I think that the both are grammatically correct, so there would be another reason for B to be the answer. Here is the URL of the forum, just in case.

http://gmatclub.com/forum/gary-s-brother-told-him

DesafinadoI think that the both are grammatically correct, so there would be another reason for B to be the answer.

I wouldn't consider (A) to be grammatically correct in standard English. I think normally I would just say "... if possible".

Yes, it could be...

Gary’s brother told him that he would attend the graduation ceremony if it were to be possible.

Gary’s brother told him that he would attend the graduation ceremony if it were to be possible.

Thanks!

A. Gary’s brother told him that he would attend the graduation ceremony if it would be possible.

B. Gary’s brother told him that he will attend the graduation ceremony if it is possible.

C. Gary’s brother told him that he will attend the graduation ceremony if it will be possible.

Consider C.
English does not permit the future modal "will" in a conditional clause.

AlpheccaStarsEnglish does not permit the future modal "will" in a conditional clause.

Generally I agree, but it seems there are certain exceptions to this, e.g. "I'll do it if it will help."