I am wondering whether the following MC test item has more than one acceptable options? The given key is A (why not?) which is completely right. How about B and C? I wanna have some expert opinions especially from native speakers of English. Thanks in advance.

—We could invite John and Barbara to the Friday night party.
—Yes,?I’ll give them a call right now. (NV 1)
A. why not B. what for
C. why D. what