Which of the following descriptions best fits physiology?

Physiology is primarily concerned with the functions of living organisms and their parts, including all physical and chemical processes. It explores how different systems within the body work to maintain homeostasis and support life. This branch of biology examines various mechanisms involved in processes such as circulation, respiration, digestion, and reproduction.

The other options refer to different aspects of biological sciences: the study of internal body structures aligns with anatomy, the relationships between body parts involve anatomical studies and their functional correlations, and changes during the life cycle pertain to developmental biology and may include aspects of physiology but are not solely focused on the functional aspects. Thus, the correct choice emphasizes the central focus of physiology on understanding the functions and processes that enable life.