I believe it’d be D. If you’re having trouble with the material, you can always take a look at previous lessons at any time, making it easier to study for an exam or something along those lines. B and C are definitely not correct, because it’s usually wrong to do so. A is also partially incorrect, because if you’re unsure of the material, saying “you only have to take quizzes” kind of sounds like “it’s easier to pass without knowing the material.”
