Final answer:
The mere exposure effect explains why Mary's friends are primarily other students from her freshman hall; the consistent time spent together during freshman year fostered familiarity and friendship.
Step-by-step explanation:
The answer that best explains why Mary's friends are other students that she spent time with on her freshman hall is C. Mere exposure effect. This psychological phenomenon suggests that people tend to develop a preference for things merely because they are familiar with them. In the context of forming relationships, this means that Mary is more likely to become friends with those she has regular contact with, such as the students in her dorm during her freshman year, as they spent a significant amount of time together in close proximity. This familiarity likely facilitated opportunities for interaction, which in turn, led to the formation of strong friendships.