**Core Concept:**
The question is testing the understanding of osmotic balance and the ions involved in maintaining extracellular fluid volume. The primary ions responsible for this are sodium (Na+), potassium (K+), chloride (Cl-), and bicarbonate (HCO3-). The balance of these ions helps in regulating body fluids and osmotic pressure.

**Why the Correct Answer is Right:**
The correct answer is **D (HCO3-)** because bicarbonate ions are the most abundant ions in extracellular fluid. They are derived from the primary source of carbon dioxide (CO2) in the body, which is primarily from cellular respiration. Bicarbonate ions are then transported into the extracellular fluid via the HCO3-/Cl- antiporter (SLC26A6) on the basolateral membrane of the renal tubule cells in the nephrons. They maintain the osmotic pressure and volume of extracellular fluid.

**Why Each Wrong Option is Incorrect:**
A. **Na+** is not the primary ion responsible for extracellular fluid volume regulation. While it is involved, its concentration is less compared to HCO3-.
B. **K+** is also not the primary ion responsible for extracellular fluid volume regulation. Its concentration is relatively low compared to HCO3-.
C. **Cl-** is also not the primary ion responsible for extracellular fluid volume regulation. Its concentration is lower than HCO3-.

**Clinical Pearl**:
Understanding the roles of various ions in maintaining extracellular fluid volume is crucial for a medical professional. A disruption in the balance of these ions can lead to various disorders like hyponatremia, hypo/hyperkalemia, and hypo/hyperchloremia. Adequate knowledge of renal physiology, particularly renal tubule functions, is essential for practicing physicians.