Pretty sure the answer is a., expanding China's trade over-seas.
He wasn't Japanese, and definitely didn't expand their religion, so it isn't b...
and he never traded with Native Americans often, so it's not d.
The only other reasonable answer would maybe be c, spreading Islam, but I had this question too and the notes I was given stated that Zheng He had a lot of ships for overseas trading, and more men (way more than Christopher Colombus).
Hope this helps!