Which of the following is the best definition of physiology?

A) the study of the structure of the human body
B) the study of the tissues and organs of the human body
C) the study of organ systems and organs of the body
D) the study of how the body functions at the various levels of organization

Physiology is the study of how the body functions at the various levels of organization, from the cellular and molecular level to the level of organ systems and the whole organism. It is concerned with the processes and mechanisms that allow living organisms to function and adapt to their environment. While anatomy is the study of the structure of the body, physiology is focused on understanding how the body works.