**Core Concept**
The immediate postoperative period is associated with a shift of potassium ions from the extracellular fluid to the intracellular fluid, leading to a decrease in serum potassium levels. This phenomenon is primarily due to the release of stress hormones, such as cortisol and adrenaline, which trigger cellular uptake of potassium.

**Why the Correct Answer is Right**
The release of stress hormones in the immediate postoperative period causes cellular uptake of potassium, leading to a decrease in serum potassium levels. This is due to the activation of the Na+/K+-ATPase pump, which pumps potassium ions into the cell in exchange for sodium ions. The increased activity of this pump results in a net decrease in serum potassium levels. Additionally, the release of aldosterone, a mineralocorticoid hormone, also contributes to potassium loss in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the changes in potassium levels in the immediate postoperative period.
**Option B:** This option is incorrect as it suggests an increase in serum potassium levels, which is not typically observed in the immediate postoperative period.
**Option C:** This option is incorrect as it does not provide a clear explanation for the changes in potassium levels in the immediate postoperative period.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor serum potassium levels closely in the immediate postoperative period, especially in patients who have undergone surgery involving the release of stress hormones or those with pre-existing kidney disease.

**Correct Answer: B. Potassium levels decrease.**