The best definition of a nutrient is b) a food component that performs a physiological function in the body.
Nutrients are substances that are essential for the growth, development, and maintenance of the body. This definition highlights that nutrients are not just food items, but specific components within food that play crucial roles in bodily functions. These components include carbohydrates, proteins, fats, vitamins, and minerals, each contributing to various physiological processes such as energy production, immune function, and muscle repair.
Other options, while related to nutrients, do not accurately capture their full significance. For example, option (a) does not account for the specific functions nutrients perform, and option (d) limits the definition to vitamins and minerals, neglecting other essential macronutrients. Therefore, option (b) provides the most comprehensive and accurate definition of what constitutes a nutrient.