While A does seem like a plausible correct answer, so is actually a conjunction and is being used as such in the sentence. However, on B, two conjunctions are being used together ("and" and "so"), which is an incorrect use of grammar. D is using correct grammar as, while not encouraged, conjunctions can be used to begin sentences. C is simply separating both complete sentences with a semicolon, making all of the sentences correct in use of grammar except for B. Hope this helps.
