Each gram of fat supplies how many kcal of energy:

**Core Concept**
The question is testing the concept of energy yield from macronutrients, specifically the energy content of fat. Fat is a macronutrient that provides energy to the body, and its energy yield is crucial for understanding nutrition and metabolism.

**Why the Correct Answer is Right**
Each gram of fat supplies 9 kcal of energy. This is because fat is a triglyceride, consisting of a glycerol backbone and three fatty acid chains. The breakdown of fat into fatty acids and glycerol releases energy, with each gram of fat yielding 9 kcal. This energy yield is a fundamental concept in nutrition and is essential for understanding the energy balance in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because fat does not yield 4 kcal per gram, which is the energy content of protein and carbohydrates.
**Option B:** This option is incorrect because fat does not yield 7 kcal per gram, which is a common misconception.
**Option C:** This option is incorrect because fat does not yield 12 kcal per gram, which is also a common mistake.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that fat is the most energy-dense macronutrient, with 9 kcal per gram, making it a crucial source of energy for the body.

**Correct Answer:** . 9 kcal.