Right answer
b. Bien. También busco una camisa.
The question in this problem is what do you think about the gray jacket? so someone wants to know how a person thinks about a jacket. Perhaps this person is a seller in a store. So the more logical answer is b, because this answer starts with the word bien that translates into it's nice. So the translation of the complete sentence is;
It's nice. I'm also looking for a shirt
