The tendency to assume that one's culture and way of life are superior to all others is termed

a. cultural relativism.

b. ethnocentrism.

c. prejudice.

d. assimilation.

Hm, I think the question here calls for "d", although d would be better defined as a focus on one's own culture, and ignoring other cultures, or seeing other cultures through the lense of own culture.

In a way, prejudice could be a good lead, but only if it said "prejudice towards all other cultures" which is not really feasible (one can't know all other cultures).

so, still, d is best, although there can be ethnocentrism without feeling superior