_________ is the least reactive metal which is used as a catalyst.

Explanation

**Step-by-Step Solution:**

1. **Identify the Category of Metals**: The question asks for the least reactive metal used as a catalyst. We need to understand which metals are considered least reactive.

2. **Understand Noble Metals**: The least reactive metals are often referred to as noble metals. These metals do not easily react with other substances and are known for their stability.

3. **List the Noble Metals**: The common noble metals include gold (Au), silver (Ag), and platinum (Pt).

4. **Determine the Least Reactive Metal**: Among these noble metals, platinum is known to be the least reactive.

5. **Define a Catalyst**: A catalyst is a substance that increases or decreases the rate of a chemical reaction without being consumed in the process.

6. **Identify the Use of Platinum**: Platinum is frequently used as a catalyst, especially in reactions involving gaseous reactants.

7. **Conclude the Answer**: Therefore, the least reactive metal that is used as a catalyst is platinum.

**Final Answer**: Platinum (Pt) is the least reactive metal which is used as a catalyst.

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