A biological catalyst is:

Options

A. a carbohydrate
B. a peptide
C. an amino acid
D. an enzyme

Explanation

To solve the question "A biological catalyst is:", we need to identify which option among the given choices (carbohydrate, peptide, amino acid, and enzyme) fits the definition of a biological catalyst.

### Step-by-Step Solution:

1. **Understand the Definition of a Catalyst**:
- A catalyst is a substance that increases the rate of a chemical reaction without undergoing any permanent change itself. It does not participate in the reaction but facilitates it.

**Hint**: Remember that catalysts are not consumed in the reaction.

2. **Differentiate Between Types of Catalysts**:
- Biological catalysts are specifically those that function in biological systems. They are typically proteins that speed up biochemical reactions.

**Hint**: Think about what substances are involved in biological processes.

3. **Analyze the Given Options**:
- **Carbohydrate**: These are organic compounds made of carbon, hydrogen, and oxygen. They serve primarily as energy sources and structural components but do not act as catalysts.
- **Peptide**: These are short chains of amino acids. While they can have biological functions, they are not specifically known as catalysts.
- **Amino Acid**: These are the building blocks of proteins. They are not catalysts themselves but are used to build enzymes.
- **Enzyme**: Enzymes are proteins that act as biological catalysts. They speed up biochemical reactions in living organisms.

**Hint**: Focus on which of these options is known for its catalytic properties in biological systems.

4. **Identify the Correct Answer**:
- Among the options, enzymes are the only substances that fit the definition of biological catalysts. They increase the rate of biochemical reactions without being consumed in the process.

**Hint**: Recall examples of enzymes like amylase, pepsin, and trypsin, which are known for their catalytic roles in digestion and metabolism.

5. **Conclusion**:
- Therefore, the correct answer to the question "A biological catalyst is:" is **enzyme**.

### Final Answer:
**Enzyme** (Option number 4 is the correct answer).