**Core Concept**
Caspases are a family of protease enzymes playing a central role in apoptosis, or programmed cell death. They are cysteine-aspartic acid proteases that cleave specific target proteins, triggering a cascade of downstream effects.

**Why the Correct Answer is Right**
Caspase activity is dependent on the presence of a cysteine residue in their active site, which forms a covalent bond with the scissile peptide bond of the target protein. This cysteine residue is essential for the catalytic activity of caspases. The active site also contains an aspartic acid residue, which helps to stabilize the transition state and facilitate the hydrolysis reaction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not relevant to the catalytic activity of caspases.
**Option B:** This option is incorrect because it is not a characteristic of the active site of caspases.
**Option C:** This option is incorrect because it is not a key component of the active site of caspases.

**Clinical Pearl / High-Yield Fact**
Caspases are responsible for the cleavage of various cellular substrates, including structural proteins, signaling molecules, and DNA repair enzymes, ultimately leading to the morphological and biochemical changes associated with apoptosis.

**Correct Answer: C. Cysteine**