............ acid is a strong oxidizing agent and so it is not used in preparation of hydrogen.

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks for an acid that is a strong oxidizing agent and is not used in the preparation of hydrogen gas.

2. **Identifying the Preparation Method:**
The common laboratory method for preparing hydrogen gas involves reacting zinc with an acid. The typical acid used for this reaction is dilute hydrochloric acid (HCl).

3. **Recognizing Strong Oxidizing Agents:**
Nitric acid (HNO3) is known to be a strong oxidizing agent. This means it can oxidize other substances easily.

4. **Analyzing the Reaction:**
When zinc reacts with nitric acid, the reaction produces zinc nitrate and hydrogen gas. However, due to the strong oxidizing nature of nitric acid, the hydrogen gas produced is further oxidized to water.

5. **Conclusion:**
Because nitric acid (HNO3) oxidizes hydrogen gas to water, it is not suitable for the preparation of hydrogen gas. Therefore, the acid that fills the blank is nitric acid.

**Final Answer:**
Nitric acid (HNO3) is a strong oxidizing agent and so it is not used in the preparation of hydrogen.

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