Assimilation is a process of adaptation and adjustment of a group of people or nations to a particular culture.
Co-operation is a process in which certain groups or individuals cooperate for the same purpose or benefits.
Conflict is a serious disagreement or an argument that can usually be long time-consuming, with no visible agreement and compromise.
Social distance means the distance and separation between certain different social groups or individuals. This distance can be on the cultural, racial, class, social, national, etc. basis.
The answer is: C.