**Core Concept**
The question is testing knowledge of arrhythmias and their relationship to the postoperative period. **Sinus tachycardia** is a common cardiac rhythm disturbance that occurs in response to stress, pain, and fluid shifts, which are commonly present in the early postoperative period.

**Why the Correct Answer is Right**
The correct answer is due to the body's response to stress and pain, which is mediated through the sympathetic nervous system. The release of catecholamines, such as adrenaline and noradrenaline, increases heart rate and contractility, leading to **sinus tachycardia**. This is a normal response to the stress of surgery and is not typically a cause for concern.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Atrial fibrillation** is more commonly seen in older patients with underlying heart disease, and is not typically the most common rhythm disturbance in the early postoperative period.
* **Option B:** **Ventricular fibrillation** is a life-threatening arrhythmia that requires immediate treatment, and is not a common rhythm disturbance in the postoperative period.
* **Option C:** **Heart block** refers to a conduction abnormality in the heart, which can occur postoperatively but is not the most common rhythm disturbance.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that **sinus tachycardia** is a normal response to stress and pain in the postoperative period, and does not necessarily require treatment unless it is causing symptoms or is associated with other cardiac complications.

**Correct Answer: C. Atrial fibrillation.**