The answer is B: Successful students study often, work hard, and turn in the assignments on time.
Parallel structure is a grammatical construction that makes each idea or item in a sentence to follow the same grammatical pattern, creating fluency, which helps the readers easily follow the writing work. In the sentence B, we can see the pattern used (only verbs) to say what successful students do: study; work; turn in.
A) doesn't have parallel structure: study; work; turning in
C) doesn't have parallel structure: study; working; to turn in
D) doesn't have parallel structure: study; to work; turning in
