Question

physiology is the study of... the actions and reactions of the body under stress. the structures of the body on a microscopic level. the functions of the structures of the body, the biochemistry, and what is actually happening.

Explanation:

Physiology focuses on the functions of body structures, biochemical processes, and the actual workings of the body. The first option is too narrow (only stress - related), and the second is about microscopic structures (which is more anatomy or histology). The third option matches the definition of physiology.

Answer:

the functions of the structures of the body, the biochemistry, and what is actually happening.