**Core Concept**
Sodium ions (Na+) play a crucial role in maintaining the resting membrane potential and facilitating the initiation and propagation of action potentials in excitable cells, such as neurons and muscle fibers. The movement of Na+ ions across cell membranes is mediated by voltage-gated sodium channels.

**Why the Correct Answer is Right**
The correct answer is related to the sodium-potassium pump (Na+/K+ ATPase), which is responsible for maintaining the concentration gradient of Na+ ions across cell membranes. This pump uses ATP to transport three Na+ ions out of the cell and two K+ ions into the cell, resulting in a higher concentration of Na+ ions outside the cell and a higher concentration of K+ ions inside the cell. This gradient is essential for the proper functioning of nerve and muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not provide any information about the sodium-potassium pump or the concentration gradient of Na+ ions.
**Option B:** Incorrect because it is unrelated to the sodium-potassium pump or the movement of Na+ ions across cell membranes.
**Option C:** Incorrect because it is a distractor that does not provide any relevant information about Na+ ions or the sodium-potassium pump.

**Clinical Pearl / High-Yield Fact**
The sodium-potassium pump is an essential mechanism for maintaining the resting membrane potential and is often targeted by cardiac glycosides, such as digoxin, in the treatment of heart failure.

**Correct Answer:** Not provided. Please provide the correct answer to complete the explanation.