The correct answer is A - that is the one sentence which has the improper use of subordinate clause, whereas all the other sentences are correct. (Because 'sniper' refers to the man, not to the gun. If it referred to the gun, all of them would be incorrect). However, C and D don't have subordinate clauses.
Still, if I had to choose one, I'd go with A - because the sentence is written incorrectly, they have to be merged into one sentence - After he fell off the boat, Rainsford swam to the island.