The correct answer is sentence (5).
A sentence fragment means that it is only a part of a sentence; it is not complete. Given that it is incomplete, the fragment, aka subordinate/dependent sentence, has to be part of a larger, independent sentence, in order to make any sense. So if you combined sentence (5) with another one, it wouldn't be a fragment anymore.
A subject and a predicate
plastic bags spread more then alot of other garbage and makes its way to sea creatures much easier, where they could be mistaken for jellyfish and eaten, which would be harmfull to the sea creature.
