**Core Concept**
Reversible cell injury is a condition where cells undergo temporary damage due to various factors, such as ischemia or hypoxia. This type of injury is characterized by distinct morphological changes that can be reversed if the underlying cause is eliminated.

**Why the Correct Answer is Right**
The correct answer is **B. Swelling of cells and organelles**. In reversible cell injury, cells undergo swelling due to the accumulation of water and ions. This is caused by an increase in the permeability of the cell membrane, leading to an influx of sodium and water into the cell. The swelling of cells and organelles is a hallmark of reversible cell injury and can be reversed if the underlying cause is eliminated, such as restoration of blood flow in the case of ischemia.

**Why Each Wrong Option is Incorrect**
**Option A:** **Necrosis** is a sign of irreversible cell injury, characterized by cell death and the breakdown of cellular structure.

**Option C:** **Karyolysis** is also a sign of irreversible cell injury, indicating the dissolution of the cell nucleus.

**Clinical Pearl / High-Yield Fact**
In reversible cell injury, the cell membrane remains intact, and the cell can recover if the underlying cause is eliminated. This is in contrast to irreversible cell injury, where the cell membrane is disrupted, and the cell undergoes necrosis.

**Correct Answer: B. Swelling of cells and organelles**