Explanation:
Letter A is the answer first because it's an effective strategy proven by studies and general knowledge. Reinforcing a certain piece of information successively within short intervals of time (while it's still 'fresh' in your mind) turns it from short-term memory into long-term memory!
Moreover, taking a look at the other options:
Letter B doesn't even make sense to what was asked by the question, as memorizing every single thing can't even be considered a strategy.
Letter C, in contradiction to letter A, has proven to be a very non-effective strategy, being even responsible for making students stressed before the exams and frustrated after them, for not achieving good results.
And finally, letter D varies from student to student. For some, it works, for others not so much. Therefore it isn't considered a general strategy.
