Final answer:
Katie's decision to choose a black shirt like her peers is an example of conformity, which is the tendency to align with group norms.
Step-by-step explanation:
Katie's action of choosing a black shirt after noticing that the three people in front of her picked black is an example of conformity. Conformity is the behavior of aligning oneself with the preferences, norms, or expectations of a group, often to fit in or to avoid standing out. In the context of social psychology, famous experiments by Solomon Asch have shown that people sometimes conform to group opinions even when they differ from their own beliefs, indicating the strong influence of social norms on individual behavior.