if it's "that apple," why wouldn't it be 사과가 instead of 사과는 since it's specific?
Subject and topic markers don't work in that way. It has nothing to do with whether something is specific or not. If it were that simple, it wouldn't be so many "how to use" articles about it. ;)
I thought we use 가 to introduce new topic? Since it's the first time we're asking about this apple, shouldn't it be 가?