Which word best describes the action of gathering into a crowd or mass?

The correct answer, which is "congregate," specifically refers to the act of coming together in a group or mass, often used in contexts involving people or animals coming together in one place. This term conveys not only the physical gathering but also sometimes suggests a purpose or intent behind the action, such as attending a meeting or a religious service.

While "assemble" also suggests gathering, it typically implies the act of bringing together for a specific purpose or activity, such as in a meeting or formation. "Disperse" describes the opposite action of scattering or spreading out, while "scattered" refers to things being spread out over a wide area or in different directions, emphasizing separation rather than gathering. Therefore, "congregate" is the most appropriate choice to convey the idea of coming together in a crowd or mass.