**Core Concept**
In the immediate postoperative period, the body's potassium balance is affected due to the stress response, fluid shifts, and changes in hormone secretion, leading to a decrease in serum potassium levels.

**Why the Correct Answer is Right**
Potassium is an electrolyte that plays a crucial role in maintaining cellular function and nerve conduction. After surgery, the body undergoes a stress response, which leads to the release of catecholamines, such as adrenaline. These hormones cause the kidneys to excrete potassium ions, resulting in a decrease in serum potassium levels. Additionally, the use of diuretics, such as furosemide, which are often administered in the postoperative period to prevent fluid overload, can further contribute to potassium loss.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the changes in body potassium levels in the immediate postoperative period.
**Option B:** This option is incorrect because it suggests that potassium levels remain unchanged, which is not the case in the postoperative period.
**Option C:** This option is incorrect because it implies that potassium levels increase, which is not supported by the physiological changes that occur after surgery.

**Clinical Pearl / High-Yield Fact**
In the postoperative period, hypokalemia (low potassium levels) is a common electrolyte imbalance that can lead to muscle weakness, arrhythmias, and cardiac arrest. It is essential to monitor potassium levels and administer potassium supplements or potassium-sparing diuretics as needed to prevent these complications.

**Correct Answer:** B. Potassium levels decrease in the immediate postoperative period.