**Core Concept**
Enzyme specificity refers to the unique ability of an enzyme to catalyze a particular chemical reaction, often due to its shape and the presence of specific amino acid residues. This specificity allows enzymes to interact with specific substrates and products, facilitating efficient biochemical pathways.

**Why the Correct Answer is Right**
The correct answer, **Option C: Active site**, is correct because the active site of an enzyme is a specific region where the substrate binds, allowing the enzyme to catalyze the reaction. The active site's shape and chemical properties are crucial for recognizing and binding the substrate, ensuring high specificity. The presence of specific amino acid residues, such as histidine, aspartate, and glutamate, also contributes to the enzyme's specificity.

**Why Each Wrong Option is Incorrect**
**Option A:** Substrate binding is not the primary determinant of enzyme specificity, although it is an essential factor.
**Option B:** Enzyme-substrate complex formation is a result of enzyme specificity, not the cause.
**Option C:** is not mentioned here to avoid repetition of correct answer, but it is correct.

**Clinical Pearl / High-Yield Fact**
A classic example of enzyme specificity is the enzyme lactase, which is responsible for breaking down lactose in the small intestine. Deficiencies in lactase lead to lactose intolerance, a common condition affecting millions worldwide.

**Correct Answer:** C. Active site.