Respuesta :
Answer: D
A: You can't say 'if the students return a book', because a bunch of students can't have one book at the same time.
B: You also can't say 'if a student returns a book...they are fined for it' because you are talking about one person, so you can't just change the number of people mid-sentence.
C: You obviously can't say 'if the students returns a book...their fined for it' because that does not make sense. It doesn't even sound right.
D: The only sensible answer here is D. Because you're talking about one person throughout.