Answer: Looking-glass self
Explanation:
Looking-glass self is defined as the theory in which as person tends to know about their characteristics and concept of self through the help of reactions gained by others. This concept develops the knowledge about self through other people.
According to the question, Peggy is using looking glass self mechanism for knowing about herself .Se is taking the reactions about herself being pretty or not from her friends.She gets the result as being pretty according to their review.