Answer: C. Jon is going to graduate in May, "although he will have to start work right away."
Explanation: A subordinate clause is a phrase that depends on another sentence, known as "The main clause", it is usually introduced by a conjunction and the subordinate clause doesn't make sense by itself. In this case, the main clause is "Jon is going to graduate in May", this sentences makes perfect sense on its own, the subordinate clause is "although he will have to start work right away", this clause is introduced by "although and it needs the main clause to make sense.
