The substance which decreases the rate of a chemical reaction in called a/an:

Options

A. inhibitor
B. poison
C. moderator
D. promoter

Answer

To solve the question regarding the substance that decreases the rate of a chemical reaction, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for a term that describes a substance which decreases the rate of a chemical reaction.

2. **Analyzing the Options**: The options provided are:
- Inhibitor
- Poison
- Moderator
- Promoter

3. **Defining Each Term**:
- **Inhibitor**: A substance that decreases the rate of a chemical reaction.
- **Poison**: A substance that can cause harm or death, often by interfering with biological processes; not specifically related to chemical reaction rates.
- **Moderator**: In the context of nuclear reactions, a moderator slows down neutrons; not directly related to chemical reactions.
- **Promoter**: A substance that increases the rate of a reaction, opposite of what we are looking for.

4. **Identifying the Correct Answer**: Based on the definitions, the term that specifically refers to a substance that decreases the rate of a chemical reaction is **inhibitor**.

5. **Conclusion**: Therefore, the correct answer to the question is **inhibitor**.

### Final Answer:
The substance which decreases the rate of a chemical reaction is called an **inhibitor**.

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