**Core Concept**
The plasma osmolality is a measure of the concentration of osmotically active particles in the blood, which can be calculated using the formula: Plasma Osmolality (mOsm/kg) = 2(Na+)(mEq/L) + (Glucose)(mg/dL)/18 + (BUN)(mg/dL)/2.8. This formula is essential for assessing the balance of electrolytes, glucose, and other solutes in the blood.

**Why the Correct Answer is Right**
To calculate the plasma osmolality, we need to substitute the given values into the formula. The plasma sodium (Na+) concentration is 125 mEq/L, glucose is 108 mg/dL, and blood urea nitrogen (BUN) is 140 mg/dL. Using the formula, we can calculate the plasma osmolality as follows: Plasma Osmolality = 2(125)(mEq/L) + (108)(mg/dL)/18 + (140)(mg/dL)/2.8.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a calculation for plasma osmolality.
**Option B:** This option is incorrect because it does not account for the correct formula or calculation for plasma osmolality.
**Option C:** This option is incorrect because it does not provide a valid or accurate calculation for plasma osmolality.

**Clinical Pearl / High-Yield Fact**
When calculating plasma osmolality, it's essential to remember that the formula includes a correction factor for glucose (mg/dL)/18 and BUN (mg/dL)/2.8 to account for the varying osmotic activity of these solutes.

**Correct Answer:** 280 mOsm/kg.