Your friend went out to drink last night, even though he knew he should have stayed in and studied for an exam. According to Freudian theory, what personality structure is causing the guilt he felt about it afterward?

The super-ego

Explanation:

According to Freudian theory, there are 3 personality structures:

The id: the most primitive of all 3 structures and the one that forms first during child development. The id is concerned with immediate gratification of needs and urges. (It's instinctive) The super-ego: It's concerned with moral rules, it's like one's conscience, or in other words "the moral compass". (it's moral) The ego: is the structure in charge of mediating with the id and the super-ego, so it's the one that keeps things in balance between the other two. (it's rational)

So, your friend went out to drink even though he should've stayed in and studied (he followed his id), now that he's feeling guilt about it, he's thinking about what he did wrong, and it's the super-ego which is concerned with what's right and what's wrong and moral rules.

Thus, the super-ego is causing the guilt.