Sentence:  Although we strongly recommend that you keep practicing until your scores top the 75% mark, 80% would be a good goal, to give yourself some margin for error in a real exam situation[…]. After you hit that point, you should be ready to <WSD>challenge</WSD> the exam. 
Meaning of the word marked with <WSD> tags:  To take (a final exam) in order to get credit for a course without taking it.