**Core Concept**
The proximate principles of nutrition, also known as the proximate composition, are the main constituents of food that provide energy and nutrients to the body. These principles include carbohydrates, proteins, fats, and fibers, which are the building blocks of a balanced diet.

**Why the Correct Answer is Right**
Carbohydrates are a primary source of energy for the body, broken down into glucose, which is then utilized by cells for various metabolic processes. They are an essential component of the proximate principles, providing approximately 4 kcal/g of energy. This is in contrast to proteins, which provide 4 kcal/g, and fats, which provide 9 kcal/g. The other options, iron, vitamin A, and calcium, are essential nutrients but are not classified as proximate principles.

**Why Each Wrong Option is Incorrect**
**Option A:** Iron is an essential mineral that plays a crucial role in oxygen transport and energy production, but it is not a proximate principle of nutrition.
**Option B:** Vitamin A is a fat-soluble vitamin essential for vision, immune function, and skin health, but it is not a proximate principle.
**Option D:** Calcium is a mineral essential for bone health and muscle function, but it is not a proximate principle.

**Clinical Pearl / High-Yield Fact**
The proximate principles of nutrition can be remembered using the mnemonic "CARBS," which stands for Carbohydrates, Amino Acids (proteins), Refined sugars, and Body fats.

**✓ Correct Answer: C. Carbohydrate**