**Core Concept**
The calorific value of a macronutrient is a measure of the energy it provides per unit mass when metabolized. In the context of nutrition, the calorific values of carbohydrates, proteins, and fats are crucial for determining the overall energy content of a diet.

**Why the Correct Answer is Right**
Protein is a macronutrient composed of amino acids, which are broken down and metabolized to produce energy. The calorific value of protein is approximately 4 kcal/g, which is similar to that of carbohydrates. This is because protein is primarily metabolized through the citric acid cycle, where it is converted into acetyl-CoA, which then enters the electron transport chain to produce ATP. The energy yield from protein metabolism is thus similar to that of carbohydrate metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the calorific value of protein is not 0 kcal/g, which would imply that it does not provide any energy.
**Option B:** Incorrect because the calorific value of protein is not 1 kcal/g, which is the value for fat.
**Option C:** Incorrect because the calorific value of protein is not 2 kcal/g, which is not a valid value for any macronutrient.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while protein provides energy, its primary role in the body is to build and repair tissues, including muscles, bones, and organs. A diet that is too high in protein can lead to an excessive energy expenditure, which may be detrimental to overall health.

**Correct Answer: C. 4 kcal/g**