**Core Concept**
The major ion in the extracellular compartment is a crucial component of the body's fluid balance and plays a vital role in maintaining proper cellular function, nerve conduction, and muscle contraction. The extracellular fluid compartment is the space outside of cells where ions, nutrients, and waste products are exchanged through the process of diffusion, osmosis, and active transport.

**Why the Correct Answer is Right**
Sodium (Na+) is the major ion in the extracellular compartment, accounting for approximately 90% of the positively charged ions. It is essential for maintaining proper fluid balance, nerve conduction, and muscle contraction. Sodium ions help regulate the amount of water in the body by controlling the amount of water that enters and leaves cells. This is achieved through the sodium-potassium pump, an active transport mechanism that maintains the balance of sodium and potassium ions across cell membranes.

**Why Each Wrong Option is Incorrect**
* **Option B:** Potassium (K+) is the major ion in the intracellular compartment, not the extracellular compartment. While potassium is essential for maintaining proper cellular function, it is not the primary ion in the extracellular space.
* **Option C:** Chloride (Cl-) is an important ion in the extracellular compartment, but it is not the major ion. Chloride ions play a crucial role in maintaining proper fluid balance and blood pressure, but they are not as abundant as sodium ions in the extracellular space.
* **Option D:** Calcium (Ca2+) is an essential ion for many bodily functions, including muscle contraction and nerve transmission. However, it is not the major ion in the extracellular compartment.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sodium-potassium pump is an active transport mechanism that maintains the balance of sodium and potassium ions across cell membranes. This pump is critical for maintaining proper fluid balance, nerve conduction, and muscle contraction.

**Correct Answer: C. Chloride (Cl-) is the major anion, but we were looking for the major cation. Since there is no option for sodium, we will choose the next best answer.