**Core Concept:** Volume overload refers to an increase in circulating blood volume, leading to impaired cardiac function and systemic congestion. Immediate post-operative period is crucial as it can lead to complications like acute kidney injury, atelectasis, and respiratory compromise.

**Why the Correct Answer is Right:** The correct answer is **C. Pulmonary Edema**. Pulmonary edema is a clinical condition characterized by the accumulation of fluid in the lungs, leading to impaired oxygenation and increased work of breathing. It is primarily caused by increased hydrostatic pressure in the pulmonary capillaries due to volume overload, which is the case immediately after an operative procedure.

**Why Each Wrong Option is Incorrect:**

A. **Hypertension:** While hypertension may complicate volume overload, it is not the earliest sign following an operative procedure. Hypertension occurs later due to the renal response to volume overload.

B. **Atelectasis:** Atelectasis is the collapse of alveoli, resulting in decreased lung expansion and oxygenation. It is a late complication of volume overload and not the immediate sign after an operative procedure.

D. **Acute Kidney Injury:** AKI is a late complication of volume overload, typically occurring after a few days of persistent congestion. It is not the immediate sign following an operative procedure.

**Clinical Pearl:** The clinical pearl here is to promptly recognize pulmonary edema as the earliest sign of post-operative volume overload and initiate appropriate treatment to prevent further complications.

**Correct Answer: C. Pulmonary Edema**

The correct answer is pulmonary edema as it is the earliest clinical sign of volume overload following an operative procedure. Recognizing this sign promptly and treating the cause (volume overload) can prevent further complications such as hypertension, atelectasis, and acute kidney injury.