**Core Concept**
Plasma osmolality is a measure of the concentration of osmotically active particles in the blood plasma. It is essential to maintain a stable plasma osmolality to ensure proper cellular function. The major contributors to plasma osmolality are sodium ions and glucose, but among the ions, one type predominates.

**Why the Correct Answer is Right**
The correct answer is sodium ions because they are the primary osmotically active particles in the blood. Sodium ions are present in large quantities in the extracellular fluid and play a crucial role in maintaining the balance of fluids within the body. The sodium-potassium pump helps regulate the concentration of sodium ions in the plasma. The high concentration of sodium ions in the plasma contributes significantly to its osmolality. The ratio of sodium ions to water in the plasma is critical in maintaining proper cellular function and preventing conditions like hyponatremia or hypernatremia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while chloride ions do contribute to plasma osmolality, their contribution is less significant compared to sodium ions.

**Option B:** This option is incorrect because potassium ions play a crucial role in maintaining cellular function and are essential for nerve and muscle function, but they are not the major contributors to plasma osmolality.

**Option C:** This option is incorrect because calcium ions are essential for various bodily functions, including muscle contraction and nerve function, but they are not the primary contributors to plasma osmolality.

**Option D:** This option is incorrect because magnesium ions are important for muscle and nerve function, but they do not significantly contribute to plasma osmolality.

**Clinical Pearl / High-Yield Fact**
Remember that the sodium-potassium pump helps maintain the balance of sodium ions in the plasma, and conditions like hyponatremia or hypernatremia can occur when this balance is disrupted. A normal plasma osmolality range is between 280-300 mOsm/kg.

**Correct Answer:** A. Sodium ions.