**Core Concept**
The maintenance of cardiac and neural function is primarily dependent on the concentration of a specific electrolyte that plays a crucial role in the generation and conduction of action potentials. The correct concentration of this electrolyte is essential to prevent arrhythmias and maintain neuromuscular function.

**Why the Correct Answer is Right**
The correct answer is related to the concentration of potassium ions (K+) in the intracellular fluid. Potassium is the primary ion responsible for the repolarization of cardiac and neural cells. During the action potential, potassium channels open, allowing K+ ions to flow out of the cell and back into the extracellular fluid, which helps to repolarize the cell membrane and prepare it for the next action potential. This process is critical for the maintenance of cardiac and neural function.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the concentration of sodium ions (Na+) is primarily responsible for the depolarization of cardiac and neural cells, not the maintenance of cardiac and neural function.
* **Option B:** This option is incorrect because the concentration of calcium ions (Ca2+) is primarily involved in the contraction of cardiac and skeletal muscle cells, not the maintenance of cardiac and neural function.
* **Option C:** This option is incorrect because the concentration of magnesium ions (Mg2+) is primarily involved in the regulation of muscle contraction and nerve conduction, but it is not responsible for the maintenance of cardiac and neural function.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that hypokalemia (low potassium levels) can lead to cardiac arrhythmias and neuromuscular weakness, while hyperkalemia (high potassium levels) can lead to cardiac arrest and respiratory paralysis. Therefore, it is essential to maintain the correct potassium concentration in patients with cardiac or neuromuscular disorders.

**Correct Answer:** C.