What happened in Vietnam after the end of colonization?

The takeover of South Vietnam by the communist North was completed on April 30, 1975, two years after the United States signed a peace treaty with Hanoi and pulled out its combat troops after a decade-long struggle. This gave the responsibility for the war to the South Vietnamese

Explanation:

the right choice would be a. culture within the city-state. c. loyalty to the police. d separation by land and sea.

its c not d   tell me if that´s correct