**Core Concept**
Rennin, also known as chymosin, is an enzyme that plays a crucial role in the digestion of milk proteins, particularly casein. It is secreted by the chief cells in the stomach lining and works in conjunction with gastric acid to break down milk into smaller peptides and amino acids.

**Why the Correct Answer is Right**
The correct answer is related to the presence of rennin in the stomach, where it acts as a gastric enzyme. Rennin works by cleaving the phosphopeptides in casein, which are then further broken down by pepsin into smaller peptides. This process is essential for the efficient digestion of milk proteins. The stomach lining secretes rennin in response to the presence of milk, allowing for the proper digestion of lactose.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because rennin is not secreted by the pancreas, which secretes enzymes that digest carbohydrates, proteins, and fats in the small intestine.
**Option B:** This option is incorrect because rennin is not present in the small intestine, where most protein digestion takes place.
**Option C:** This option is incorrect because rennin is not secreted by the salivary glands, which produce enzymes that break down carbohydrates in the mouth.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that rennin plays a crucial role in the digestion of milk proteins, particularly in infants and young children who consume milk as a primary source of nutrition. In individuals with lactose intolerance or milk protein allergy, the presence of rennin can exacerbate symptoms.

**Correct Answer: D. Stomach lining**