(C) Assessment of Body Composition
BMI (Body Mass Index) is used to assess body composition — specifically whether a person is underweight, normal, overweight, or obese, based on weight relative to height.
The passage (Question 13 in Exercises) uses BMI < 18.5 kg/m² to identify adults who are undernourished, confirming BMI measures body composition. Option (D) is a partial use of BMI, not its definition. Examiners expect the standard definition: BMI assesses body composition.