A belief that one's culture is superior to another

a. Ethnocentrism

b. Prejudice

c. Sterotype

d. Racism

The belief is focused on a culture: this means that it is not focuced on race for example. (so answer c is wrong).

The correct answer is a. Ethnocentrism. - means having own ethnicity (culture) in the center.

Stereotypes and Prejudices are rather beliefs about other cultures, not one's own.