Eric is very hungry, but instead of cutting the line in the cafeteria, he decides to wait in line to get food. which part of his personality is dominating his behavior at the present moment

I believe the part of his personality which is dominating his behavior at the present moment is ego.

It is not id, because id refers to our basic needs - and his basic need is for food, which he didn't satisfy given that he decided not to cut the line. It isn't superego either, because that is unconscious, and he consciously made his decision. It is not preconscious either, because that refers to your recollection of forgotten memories.