Final answer:
A psychologist investigating how teens are influenced by their peers is most likely to be in the subfield of social psychology.
Step-by-step explanation:
The psychologist investigating how teens are influenced by their peers would most likely be in the subfield of social psychology. Social psychology focuses on how the presence of others impacts a person's behavior and reactions, including the influence of peers on individual behavior. Social psychologists study the composition of social groups and examine how behavior is influenced by who else is present in a given situation.