Sarah is torn between the need to study for an exam and her desire to go out with her friends. She decides that she will go out later only if she completes her studying. This realistic decision reflects the functioning of Sarah's

ego

Explanation:

According to Freud, the ego is the part of the personality that mediates the demands of the id, the superego, and reality. In this sense, id presses us from within telling us what we want to do and the superego conditions us from the outside reminding us what we should do. Sarah's ego will control the desire to leave with her friends, reaching the agreement that she will only go outside if she finishes her studies.