I thought, "do you drink milk at breakfast?" could be accepted as well.

The answer given, "for breakfast", suggests that they only drink milk at breakfast time and don't eat anything. Rather than, "at breakfast" which would mean they might also eat other things. So I don't know how you distinguish between these details in Ukrainian.

Agreed

What is the rule for п'є, п' єш, п'ємо and п'ють???