Final answer:
A researcher studying how each person defines themselves is examining self-concept, a central concept in social psychology concerned with individuals' identities as formed through social interactions.
Step-by-step explanation:
A researcher who is interested in how each person defines himself or herself, or themselves, is studying self-concept. Self-concept is a fundamental aspect of social psychology that involves an individual's perception of themselves that is developed through social interaction. We understand our own identity in part by our perception of how others view us, a notion famously termed as "the looking glass self" by sociologist Charles Cooley, and further elaborated on by George Herbert Mead. The development of the self is seen as being significantly influenced by social processes and situational factors within an individual's environment.
The other options provided, such as social perception, social influence, and group dynamics, are also central concepts in social psychology but they do not specifically focus on how individuals define their own identities. Social perception relates more to how we see others, social influence revolves around how others' behaviors influence our own, and group dynamics involves how people behave and interact within groups.