Which of the following is called "biological middleman''

Options

A. Hormone
B. Vitamin
C. Enzyme
D. All the above

Explanation

**Step-by-Step Solution:**

1. **Understanding the Terms**: We need to clarify what hormones, vitamins, and enzymes are. Hormones are chemical messengers that regulate various functions in the body. Vitamins are organic compounds that are essential for normal growth and nutrition. Enzymes are proteins that act as catalysts to speed up chemical reactions in the body.

2. **Identifying the Role of Each**:
- **Hormones**: They facilitate communication between different parts of the body and regulate physiological processes but do not act as intermediaries in chemical reactions.
- **Vitamins**: They are essential for various bodily functions but do not directly participate in chemical reactions as catalysts.
- **Enzymes**: They are biological catalysts that increase the rate of chemical reactions without being consumed in the process. They facilitate the conversion of substrates into products.

3. **Defining "Biological Middleman"**: The term "biological middleman" refers to a substance that facilitates a process without being altered or consumed in that process.

4. **Conclusion**: Since enzymes act as catalysts in biochemical reactions, facilitating the conversion of reactants into products without being consumed or changed themselves, they fit the definition of a "biological middleman." Therefore, the answer to the question is **Enzymes**.

**Final Answer**: Enzymes are called the "biological middleman."

---