When eighteen year old Aaron received a large inheritance from his grandfather, he was tempted to purchase a sports car. Aaron decided, instead, to deposit his inheritance into an investment account for his college education. Aaron demonstrates a:

Aaron demonstrates a strong ego.

Ego, according to Freud, refers to rational thoughts - here, Aaron realized that what was best in his interest was to invest the money into his college account so as avoid college debts in the future. Id refers to instinctual drives, which would be his rash decision to buy a sports car. Locus of control refers to the fact that we either have control over our lives, or don't, and cannot be applied here. So, obviously, a strong ego is the correct answer.