Final answer:
Body weight is a multifactorial trait because it is influenced by multiple genetic and environmental factors, making option b) the correct answer. It is controlled by polygenic traits, with gene variability and environmental influences such as diet and exercise playing vital roles.
Step-by-step explanation:
Body weight is indeed a multifactorial trait, meaning that it is not determined by a single gene. Instead, body weight is influenced by multiple factors, both genetic and environmental. This complex interaction factors in gene variability, diet, exercise, socioeconomic factors, and more, contributing to one's overall body weight. Traits such as body weight that show a wide range and variability and can be measured are generally considered to be polygenic traits. These traits are controlled by more than one gene, and each gene may have several alleles, adding to the complexity. The influence of environmental factors, such as nutrition and physical activity, is also substantial in determining body weight. It's essential to recognize that lifestyle choices and social circumstances can directly or indirectly impact a person's weight.
Given this information, the correct option for why body weight must be a multifactorial trait is b) It is influenced by multiple genetic and environmental factors. This acknowledges the role that both genetics and the environment play in determining body weight.