When julie's boyfriend bought her a nice present, she thought, "he buys me presents because hes such a nice person. this kind of explanation is referred to as?

The correct answer is attribution.

Attribution is a term in psychology which refers to the act of explaining a particular thing or event. In the case above, Julie is trying to give a cause as to why her boyfriend bought her a present, and she came up with the reason which is because he is a nice person. She attributed his action with his characteristics.