**Core Concept**
Apoptosis is a form of programmed cell death characterized by distinct morphological changes that can be visualized under light microscopy. It is a vital cellular process that eliminates damaged or unwanted cells, thereby maintaining tissue homeostasis and preventing disease.

**Why the Correct Answer is Right**
During apoptosis, the cell undergoes a series of changes, including shrinkage, chromatin condensation, and membrane blebbing. The characteristic feature of apoptosis on light microscopy is the presence of apoptotic bodies, which are small, membrane-bound vesicles containing fragmented cellular components. These apoptotic bodies are generated as a result of the activation of caspases, a family of cysteine proteases that play a central role in the execution of apoptosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Karyolysis is a feature of necrosis, not apoptosis. Karyolysis refers to the dissolution of the nucleus, which is a hallmark of cell death due to necrosis or infection.

**Option B:** Pyknosis is a feature of both apoptosis and necrosis, but it is not the most characteristic feature of apoptosis on light microscopy. Pyknosis refers to the condensation of chromatin, which can occur in both apoptotic and necrotic cells.

**Option C:** Cell swelling is a feature of necrosis, not apoptosis. Cell swelling occurs due to the disruption of cellular ion and water balance, leading to cell lysis and death.

**Clinical Pearl / High-Yield Fact**
Remember that apoptosis is a distinct process from necrosis, and recognizing the characteristic features of apoptosis is crucial in diagnosing and managing various diseases, including cancer and autoimmune disorders.

**Correct Answer: D. Apoptotic bodies**