**Core Concept**
Cell injury is a critical concept in pathology and pharmacology, referring to the damage or stress that cells undergo in response to various insults such as toxins, infections, or ischemia. The type and severity of cell injury can vary, and understanding the mechanisms and progression of cell injury is essential for diagnosis and treatment.

**Why the Correct Answer is Right**
The earliest and most common form of reversible cell injury is cellular swelling, also known as hydropic change. This occurs due to an imbalance in the ion and water transport across the cell membrane, leading to an influx of water and ions into the cell. As a result, the cell swells and becomes distended, but the cell membrane remains intact, allowing for potential recovery. The sodium-potassium pump plays a crucial role in maintaining ion balance and preventing excessive water entry.

**Why Each Wrong Option is Incorrect**
* **Option A:** Necrosis is a form of irreversible cell injury characterized by cell death and the breakdown of cellular structures. It is not a reversible process.
* **Option B:** Apoptosis is a form of programmed cell death, which is a distinct process from reversible cell injury. While apoptosis can be triggered by various insults, it is not the earliest and most common form of reversible cell injury.
* **Option C:** Liquefactive necrosis is a type of necrosis that involves the breakdown of cellular structures and the accumulation of dead cells. It is not a reversible process.

**Clinical Pearl / High-Yield Fact**
Reversible cell injury can progress to irreversible cell injury if left untreated or if the underlying insult persists. Recognizing the early signs of reversible cell injury is crucial for timely intervention and potential recovery.

**Correct Answer: C. Cellular swelling**