Which nutrient serves as a major energy source and is commonly found in whole grains, fruits, and vegetables?

The correct answer identifies carbohydrates as the nutrient that serves as a major energy source. Carbohydrates are crucial for providing energy to the body, particularly because they are readily broken down into glucose, which is then utilized by the body's cells. They are predominantly found in foods such as whole grains, fruits, and vegetables. These food sources not only offer a significant amount of carbohydrates but also come with added benefits like fiber, vitamins, and minerals, making them important for overall health.

In contrast, proteins primarily function as building blocks for tissues, muscles, and enzymes rather than serving as a primary energy source. Fats are also energy sources and provide more energy per gram than carbohydrates but are not the body's first choice for quick energy, particularly during high-intensity activities. Vitamins are essential nutrients but do not themselves provide energy; rather, they support various biochemical functions in the body. Therefore, carbohydrates are distinctly recognized for their role in energy production and align with the foods mentioned in the question.