A is the fragment. "Because...night" is not a sentence. It is a dependent clause. You can attach it to a predicate and explain what happened when he walked at night, but the predicate isn't given. Since you only have an independent cause, which is given away by the word because, this is a fragment. B, C, and D have subject, verb, AND predicate/no dependent clause. You have to have a dependent clause (a fragment sentence) attached to an independent (an actual sentence). I know this is lengthy but I hope it helps.
