Final answer:
A person's weight is determined by heredity, activity level, and body composition, while body mass index (BMI) is a measure of weight relative to height and categorizes individuals into weight classes.
Step-by-step explanation:
The factors that play a role in determining a person's weight include heredity, activity level, body mass index (BMI), and body composition. Heredity refers to the genetic factors passed down from parents that can influence body size, shape, and metabolism.
Activity level impacts caloric expenditure and therefore affects weight. BMI is a measure of weight in relation to height and is used to categorize individuals as underweight, normal weight, overweight, or obese. However, it does not directly determine a person's weight but rather is a result of one's height and weight. Lastly, body composition, the relative amount of muscle, fat, bone, and other vital parts of the body, directly affects a person's weight and overall health.