**Core Concept**
The most abundant extracellular ion in the human body is a crucial aspect of maintaining proper fluid balance and electrical neutrality. This ion plays a vital role in various bodily functions, including nerve conduction, muscle contraction, and maintaining the resting membrane potential.

**Why the Correct Answer is Right**
The correct answer is sodium (Na+). Sodium is the most abundant extracellular ion, with an average concentration of around 135-145 mmol/L in the extracellular fluid. This is due to the high permeability of the epithelial cells in the distal convoluted tubule and collecting ducts of the kidneys, which allows for the reabsorption of sodium ions back into the bloodstream. The sodium-potassium pump (Na+/K+-ATPase) also contributes to maintaining the concentration gradient of sodium ions across cell membranes.

**Why Each Wrong Option is Incorrect**
* **Option B:** Potassium (K+) is the most abundant intracellular ion, not extracellular ion.
* **Option C:** Chloride (Cl-) is the most abundant anion in the extracellular fluid, but it is not the most abundant ion overall.
* **Option D:** Calcium (Ca2+) is an essential ion for various bodily functions, but it is not the most abundant extracellular ion.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the balance of sodium and water in the body is crucial for maintaining proper blood pressure and preventing conditions such as hyponatremia and hypernatremia.

**Correct Answer: C. Chloride.**