Body mass index (BMI) is primarily used to assess what?

Body Mass Index (BMI) is primarily utilized to evaluate the relative weight in relation to height and serves as a predictor of future disease risk. It is calculated using a person's weight in kilograms divided by the square of their height in meters. This simple formula provides a numerical value that classifies individuals into categories such as underweight, normal weight, overweight, and obesity.

The reason this classification is important is that it correlates with the risk of developing various health issues, such as heart disease, diabetes, and certain cancers. Higher BMI values can indicate a greater likelihood of these health problems, thus offering insights into the potential future health risks of an individual.

The other options, while related to aspects of health and fitness, do not capture the primary use of BMI. Overall body composition involves more detailed analysis, including muscle mass and fat distribution, which BMI does not provide. Similarly, body fat percentage is a more specific measurement that directly assesses fat rather than simply comparing weight to height. Assessing fitness level encompasses a wider range of factors, including cardiovascular endurance, muscular strength, and flexibility, which BMI does not adequately reflect. Therefore, the focus of BMI on weight, height, and disease risk makes option D the most accurate representation of its primary use.