选D.这里是要按主句的,没错.
但不是所有情况下都是按主句的,具体是根据逻辑确定到底要对什么进行反问.就这个句子的逻辑来讲,上面的说话的人和听的人都应该知道he left his English这件事的,所以不需要反问这个.但是如果句子改成这样就不同了：
I think he has left his English.
就这里的逻辑来讲,就不能跟随主句了,你不需要反问你是不是这样认为的,所以应该是：
I think he has left his English,hasn't he?
如果改成这样：
He thought she had left her English.就跟随主句或从句都符合逻辑了.试比较：
He thought she had left her English,didn't he?(他不是这样认为吗?）
He thought she had left her English,hadn't she?(她没有放弃英语吗?)