Which of the following is an example of a macronutrient?

Protein is categorized as a macronutrient because it is one of the three main nutrients that provide energy and are essential for body function. Macronutrients include carbohydrates, fats, and proteins, which the body requires in larger amounts compared to micronutrients like vitamins and minerals.

Carbohydrates and fats, alongside proteins, are crucial for providing energy, supporting growth, and maintaining health. Proteins, in particular, play a vital role in the body's structure and function, as they are the building blocks for muscles, tissues, and enzymes.

The other options refer to micronutrients: vitamin C and calcium are essential for various bodily functions but are needed in smaller amounts. Iron, while important for oxygen transport in the blood, is also classified as a micronutrient. Therefore, protein is the only choice that fits the definition of a macronutrient.