## Core Concept
The question tests the understanding of high-energy phosphate compounds and their role in energy transfer within cells, specifically focusing on the replenishment of ATP (adenosine triphosphate), which is a crucial energy carrier in cellular processes.

## Why the Correct Answer is Right
Phosphocreatine (or creatine phosphate) is a high-energy compound found in muscle cells. It acts as a rapid reservoir for ATP formation during high-intensity, short-duration activities. When ATP is depleted, phosphocreatine donates a phosphate group to ADP (adenosine diphosphate) to quickly regenerate ATP through the enzyme creatine kinase, thereby maintaining energy supply.

## Why Each Wrong Option is Incorrect
- **Option A:** While NADH and FADH2 are crucial in the electron transport chain for generating ATP, they are not considered high-energy phosphates but rather electron carriers.
- **Option B:** GTP (Guanosine triphosphate) is indeed a high-energy phosphate compound but is primarily involved in biosynthesis and signal transduction rather than serving as a direct reservoir for ATP replenishment.
- **Option C:** ATP itself is the primary energy currency of the cell but does not act as a reservoir for its own formation; rather, it is the end product of energy conversion processes.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the phosphocreatine kinase system is vital for providing a rapid source of ATP during the initial stages of intense muscle activity. This system is particularly important in skeletal and cardiac muscles, which have high and variable energy demands.

## Correct Answer: D. Phosphocreatine.