**Core Concept:** Energy reserve of the body refers to the storage of nutrients that can be readily converted into usable energy when needed. The main energy reserve in humans are glycogen in the liver and muscles, fat, and proteins.

**Why the Correct Answer is Right:** The correct answer is **D:** Glycogen, as it is the primary energy reserve in the body, mainly stored in the liver and muscles. Glycogen is formed from glucose and serves as a readily available source of energy when glucose levels are low.

**Why Each Wrong Option is Incorrect:**

A. Fat (Option A) is a secondary energy reserve, primarily used when glycogen levels are depleted. However, it is not the primary reserve.

B. Protein (Option B) is used as a third energy reserve, but it is not the primary reserve. Protein energy reserves are utilized when glycogen and fat stores are depleted.

C. None of the options provided (Option C) is incorrect, as the correct answer is indeed D (glycogen).

**Clinical Pearl:** In critical situations like low blood glucose levels, the body can convert glycogen to glucose through a process called gluconeogenesis. This is essential for maintaining blood glucose levels and preventing hypoglycemia.

**Correct Answer: D. Glycogen**