Body composition refers to:

The definition of body composition encompasses both the fat and fat-free components of the body, making option C the correct choice. Body composition is a crucial aspect of health and fitness that represents the different types of tissues that make up the human body. This includes the amount of body fat, which is the fat component, and the fat-free mass, which consists of muscles, bones, and organs.

Understanding body composition provides a more detailed insight into a person's health than simply looking at total body weight. For instance, two individuals might weigh the same but have very different body compositions, one with higher muscle mass and the other with more fat. This differentiation is integral in fields such as fitness training and health assessments, where the focus is often on improving body composition for better health outcomes.

The other options do not fully encapsulate the concept of body composition. Merely stating muscle mass overlooks important components such as fat and organs. A simple ratio of weight to height refers to the Body Mass Index (BMI), which does not differentiate between lean and fat mass, thus failing to provide a complete picture. Total mass alone does not address the composition of the mass and is insufficient for evaluating health and fitness. Therefore, option C accurately reflects the comprehensive nature of body composition.