Answer:
Your answer would be A.
Step-by-step explanation:
You should place a comma before so because it is followed by an independent clause. So is one of the seven coordinating conjunctions represented by the mnemonic FANBOYS (for, and, nor, but, or, yet, so). when these coordinating conjunctions link two independent clauses, the conjunction is preceded by a comma. What is more, you could replace so with therefore in order to be sure whether to add a comma or not. If the sentence works with the replacement and the meaning is not altered, then so is a coordinating conjunction and should have a comma.