**Core Concept**
Curdling of milk is a process where milk is coagulated, resulting in the separation of solid curds from liquid whey. This process is primarily caused by the action of an enzyme that catalyzes the conversion of casein proteins into a gel-like substance.

**Why the Correct Answer is Right**
The correct answer is **Rennet**. Rennet is a complex of enzymes that is produced in the stomachs of ruminant mammals, such as cows. It contains the enzyme chymosin, which specifically cleaves the kappa-casein protein in milk, causing the casein micelles to aggregate and form a gel-like network. This process is essential for the digestion of milk in ruminants and is also used to produce cheese.

**Why Each Wrong Option is Incorrect**
**Option A:** **Pepsin** is an enzyme that is involved in the digestion of proteins in the stomach, but it is not responsible for the curdling of milk.
**Option B:** **Lactase** is an enzyme that breaks down lactose, a sugar found in milk, but it does not contribute to the curdling process.
**Option C:** **Trypsin** is an enzyme that breaks down proteins in the small intestine, but it is not involved in the curdling of milk.

**Clinical Pearl / High-Yield Fact**
Rennet is a key ingredient in the production of cheese, and its action is responsible for the characteristic texture and flavor of many types of cheese.

**Correct Answer: C. Rennet**