The answer that shows the use of an apostrophe to indicate possession is "firefly's." This is because it implies that something IS, something BELONGS TO the firefly. Ex. The firefly's wings were glowing.
'86 and "won't" do not showing possession because they are only abbreviations, an abbreviation for 1986 and one for will not.
A's and B's are not examples either because they simply use the proper plural for singular letters. In english, while discussing how many A's there are in the word extravagant, we wouldn't say " there are three As," but "there are three A's."
