**Core Concept**
Normal plasma osmolality is the balance between the concentration of solutes in the plasma and the solvent (water). It is essential for maintaining proper fluid balance and preventing dehydration or hyponatremia.

**Why the Correct Answer is Right**
The normal plasma osmolality is approximately 285-295 mOsmol/kg H2O. This range is maintained by the balance of sodium, chloride, and other solutes in the plasma. The osmoregulatory system, which involves the hypothalamus, pituitary gland, and kidneys, plays a crucial role in regulating plasma osmolality. When plasma osmolality increases or decreases, the hypothalamus stimulates the release of antidiuretic hormone (ADH) or vasopressin, which in turn affects water reabsorption in the kidneys.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is too low. A plasma osmolality of 300 mOsmol/kg H2O would indicate hypernatremia.
**Option C:** Incorrect because it is an undefined value. Plasma osmolality should be expressed in mOsmol/kg H2O.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the causes of hyponatremia is "SIADH" (Syndrome of Inappropriate Antidiuretic Hormone Secretion), which accounts for approximately 15% of all cases of hyponatremia.

**Correct Answer: D. 285-295**