**Core Concept**
Post-cholecystectomy hyponatremia is most commonly due to syndrome of inappropriate antidiuretic hormone secretion (SIADH), especially in the early postoperative period. SIADH leads to water retention and dilutional hyponatremia, with serum sodium <135 mEq/L and normal or low urine osmolality.

**Why the Correct Answer is Right**
In this patient, serum sodium is 125 mEq/L, indicating mild to moderate hyponatremia. Given that she is asymptomatic and the hyponatremia is likely due to SIADH post-surgery, the first-line management is **free water restriction**. This helps prevent further dilution of serum sodium by reducing water intake. Sodium levels typically rise slowly over days with this approach. Administration of hypertonic saline (Option A) is reserved for severe, symptomatic hyponatremia (e.g., seizures), and may worsen cerebral edema if used inappropriately. Plasma ultrafiltration and hemodialysis are invasive and indicated only in severe, refractory cases with rapid onset of neurological symptoms.

**Why Each Wrong Option is Incorrect**
Option A: Hypertonic saline is contraindicated in mild hyponatremia and can lead to osmotic demyelination if used inappropriately.
Option C: Plasma ultrafiltration is used in severe, life-threatening hyponatremia with rapid onset, not in asymptomatic patients.
Option D: Hemodialysis is reserved for end-stage renal disease or severe, intractable hyponatremia with acute neurological deterioration.

**Clinical Pearl / High-Yield Fact**
In asymptomatic hyponatremia after surgery, especially in middle-aged women, SIADH is the most common cause. Free water restriction is the first-line treatment, with close monitoring of sodium levels. Never administer hypertonic saline without confirming severe symptoms or neurological deficits.

✓ Correct Answer: B. Restriction of free water