What was Ben's major in The Graduate?

I don't believe it's ever really intended for us to know what Ben majored in while he was in college. He clearly doesn't seem to care that he's earned his degree, and can't figure out what to do with himself afterwards, so not telling us and showing that Ben doesn't seem to care in turn suggests that we shouldn't care either. This is further evidenced by the fact that they never even name what college Ben went to, simply referring to it as a college, "Back east."

Could also be the writers simply forgot to mention it.

Knowing isn't really central or pivotal to the plot or point of the film.

While no specific Major was ever mentioned, Benjamin was the Editor of the student newspaper. This indicates he was probably in the Journalism realm, though that's by no means definite. It is also discussed that he is a Track star and that his college was "back East", but other than that we are not really told anything else about his college life.

A really thorough analysis of the entire film, probably more than you care to know, can be found here.