Final answer:
Cecily's careful negotiation of privacy boundaries at her new high school illustrates the Symbolic Interactionist Theory, which focuses on individual relationships and communication within society.
Step-by-step explanation:
Cecily, who is new to her high school and is careful about whom she shares information with, is exhibiting behavior that aligns with the Symbolic Interactionist Theory. This sociological theory is a micro-level perspective focusing on the relationships among individuals within a society and the significance of communication, including the negotiation of boundaries and privacy. This theory posits that individuals are active in shaping their social world, using language and symbols to make sense of it and interact with others.