**Core Concept**
The Na+ - K+ pump is a crucial ion pump that maintains the electrochemical gradient across the cell membrane by actively transporting sodium ions out of the cell and potassium ions into the cell, using ATP hydrolysis as an energy source.

**Why the Correct Answer is Right**
The correct answer involves the understanding of the mechanism of the Na+ - K+ pump, a type of P-type ATPase enzyme. This enzyme pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell against their concentration gradients, using the energy from ATP hydrolysis. The pump's activity is essential for maintaining the resting membrane potential, regulating intracellular and extracellular ion concentrations, and facilitating nerve and muscle function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Na+ - K+ pump does not directly regulate calcium ion concentrations.
**Option B:** This option is incorrect because the Na+ - K+ pump does not use GTP as an energy source; instead, it relies on ATP hydrolysis.
**Option C:** This option is incorrect because the Na+ - K+ pump does not have a direct role in the regulation of pH levels within the cell.

**Clinical Pearl / High-Yield Fact**
The Na+ - K+ pump is an essential component of the body's sodium-potassium pump mechanism, and its dysfunction can lead to conditions such as hypertension, cardiac arrhythmias, and muscle weakness.

**Correct Answer:** C. It uses ATP as a source of energy.