still confused

Hi Kenken, thanks for the question. The passage here states the following: If taught with appropriate methods AND devote significant effort, then achieve broad mastery. The conclusion states: If not achieve broad mastery, then not taught with appropriate methods. Answer choice A would fully justify the conclusion because if being taught with appropriate methods is sufficient on its own to achieve broad mastery, then it would hold that not achieving broad mastery is sufficient for not being taught with appropriate methods (the contrapositive of the answer choice). I hope this helps. Please let me know if you have any other questions.