I believe it is D because it's not a,b and I'm pretty sure it's not c
The Khmer Rouge in Cambodia was primarily a result of an extreme political ideology. Cambodians have Vietnamese influence and had desire to kick the French out. When their civilization suffers, they resorted to extremism.
The answer would be letter A.
the soviet union broke into 15 sovereign states
Manifest Destiny is what you are looking for
hope it helps