Define the term "metabolism."

The term "metabolism" encompasses all biochemical processes that occur within the body, which includes the conversion of food into energy, as well as the synthesis and breakdown of various substances necessary for bodily function. This definition is broad and captures both catabolic processes (breaking down substances for energy) and anabolic processes (building new compounds for growth, repair, and maintenance).

By understanding metabolism in this comprehensive way, one appreciates that it involves not just energy production from food but also the myriad of reactions that sustain life, from cellular repair to the regulation of hormones. Therefore, the correct choice accurately reflects the complexity and indivisibility of metabolic processes within the human body, highlighting its essential role in overall health and functioning.

In contrast, the other options represent limited or incomplete views of metabolism. While option A refers to energy expenditure specifically at rest, it does not account for the full range of metabolic activities. Option C narrows down metabolism to just the conversion of food into energy, overlooking many critical processes like biosynthesis and waste elimination. Option D restricts the focus to the breakdown of fats and proteins, ignoring carbohydrates and other vital metabolic pathways. Thus, the chosen answer is correct as it encapsulates the entirety of metabolism.