Answer: "Nunca"
Explanation: Gloria no es una estudiante muy buen, nunca estudia.
Translation: Gloria is not a very good student, she never studies.
For the sentence to have a grammatical meaning, it is necessary to place an adverb of time, so the words "muy" and "mucho" are discarded. The right word ends up being "nunca" since we want to explain why Gloria is a bad student. If Gloria always studied hard, she wouldn't be a bad student.