**Core Concept**
The sodium-potassium pump (Na+ - K+ pump) is an essential enzyme that maintains the electrochemical gradient across the cell membrane by actively transporting sodium ions out and potassium ions into the cell. The pump has two main binding sites: one for sodium ions (extracellular) and another for potassium ions (intracellular).

**Why the Correct Answer is Right**
The correct answer is a site located on the intracellular side of the Na+ - K+ pump, specifically the binding site for potassium ions. This site is crucial for the pump's function, as it allows potassium ions to bind and be transported into the cell. The binding site is composed of a specific region on the pump's intracellular domain, which interacts with the potassium ion.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because it is not specified as an intracellular binding site.
* **Option B:** Incorrect because it is not related to the Na+ - K+ pump.
* **Option C:** Incorrect because it is not a recognized binding site on the Na+ - K+ pump.

**Clinical Pearl / High-Yield Fact**
The Na+ - K+ pump is an essential enzyme for maintaining the resting membrane potential and is often targeted by certain medications, such as cardiac glycosides, which can inhibit the pump and lead to arrhythmias.

**Correct Answer: C. Intra-cellular binding site for potassium ions.**