Which of the following is not a subfield of physics?

Sociology is not a subfield of physics because it is a social science that focuses on the study of human behavior, social relationships, and societal structures. In contrast, the other options—astronomy, optics, and quantum mechanics—are all core areas within the field of physics.

Astronomy involves the study of celestial objects and phenomena beyond Earth's atmosphere, while optics deals with the behavior and properties of light. Quantum mechanics is a branch of physics focused on the behavior of matter and energy at very small scales, such as atoms and subatomic particles. Each of these disciplines utilizes principles of physics to explore and explain natural phenomena, which is why they are considered subfields of the subject, unlike sociology.