Final answer:
Children classifying others as "people who are nice" or "people who are mean" are exhibiting social categorization, which is distinct from Piagetian concepts like egocentrism or concrete operational thought.
Step-by-step explanation:
Children who are able to classify other people into groups such as "people who are nice" and "people who are mean" are demonstrating social categorization. This is part of their social cognitive development, through which they organize information about people and form generalizations. While the other options are related to cognitive development stages identified by Piaget, social categorization is specifically about children recognizing groups and classifying people based on perceived characteristics, which is different from egocentrism (a focus on self), concrete operational thought (logical thinking about real events), or centration (focusing on one aspect of a situation while ignoring others).