What is physiology the study of?

Physiology is defined as the study of the functions and processes of the body and its systems. While the correct answer acknowledges the body's structure, it is important to clarify that physiology specifically relates to how these structures function rather than just what they are. It encompasses the biological mechanisms and processes that enable the body to maintain life, respond to challenges, and perform complex activities.

The other options focus on different areas: human evolution pertains to developmental biology and anthropology, the effects of drugs on the body are typically studied under pharmacology, and interpersonal relationships in healthcare relate to communication and psychology within healthcare settings. None of these capture the essence of physiology, which is fundamentally about the functioning of biological systems rather than their structure or the external influences acting upon them.