Provides an explanation of why people belong to informal groups.

The correct answer is "the Social Identity Theory"

The Social Identity Theory is depicted as a hypothesis that predicts certain intergroup practices based on apparent gathering status contrasts, the apparent authenticity and strength of those status contrasts, and the apparent capacity to move from one group to another. This stands out from events where the expression "Social Identity Theory" is utilized to allude to general hypothesizing about human social selves.