The correct answer is B. evaluate a work in terms of its beauty.
An art historian studies and explains the context, the origins, the meaning, the influences, the consequences of a work of art. He or she doesn't just say that something is more or less beautiful. That would be an amateurish, impressionistic judgment about a work of art. He or she tries to explain why something might be considered beautiful in a certain age. He or she is also interested in the very concept of beauty - how people see it, how it changes throughout the centuries, etc. - but also in the concept of "ugliness", why and how does it work in art.