I would say:
 c. The Vietnamese wanted to be free of all foreign influence.
 d. U.S. forces fought a conventional war, while their communist foes conducted a guerilla campaign.
 I do not think that: b. Once South Vietnam surrendered, the rest of Southeast Asia soon fell to communist invaders, is true. If, for example, Cambodia, adopted a communist regime (and almost immediately started a war against Vietnam), Thailand, on the other hand, didn't.