A biological catalyst is

Options

A. an amino acid
B. an enzyme
C. a carbohydrate
D. the `N_2` molecule

Explanation

### Step-by-Step Solution:

1. **Understanding the Term "Biological Catalyst":**
- A biological catalyst is a substance that speeds up a chemical reaction in living organisms without being consumed in the process.

2. **Identifying the Options:**
- The options provided are:
- Amino acid
- Enzyme
- Carbohydrate
- N2 (Nitrogen gas)

3. **Analyzing Each Option:**
- **Amino Acid:** While amino acids are the building blocks of proteins and can play a role in biological processes, they are not catalysts themselves.
- **Enzyme:** Enzymes are proteins that act as biological catalysts, facilitating and speeding up biochemical reactions in the body.
- **Carbohydrate:** Carbohydrates are primarily energy sources and structural components, not catalysts.
- **N2 (Nitrogen gas):** Nitrogen gas is not a catalyst and does not participate in biological reactions as a catalyst.

4. **Conclusion:**
- Based on the analysis, the only option that fits the definition of a biological catalyst is **Enzyme**. Enzymes are crucial for various biochemical reactions, such as digestion and metabolism, and they operate under conditions that are compatible with life.

5. **Final Answer:**
- The correct answer is **Option 2: Enzyme**.