Final answer:
Waist circumference is the least important factor in determining energy requirements for activity; intensity, weight, and exercise duration are more significant.
Step-by-step explanation:
The question addresses the factors that determine the total amount of energy a person requires for activity. Among the options provided, (a) waist circumference is the least important factor in determining the total amount of energy a person requires for activity. The most significant factors are (b) intensity of the activity, (c) their weight, and (d) amount of time spent exercising. These aspects directly affect how much energy the body needs as they determine the extent to which the body's energy resources are being used.
Waist circumference does not have a direct impact on the amount of energy used during physical activity. Instead, factors such as the intensity and duration of the activity, as well as body weight (which can imply muscle mass and how much effort is needed to move the body), play a far more critical role in energy demands for various activities. For athletes or individuals engaged in regular physical activity, energy needs can be significantly higher due to increased energy expenditure.