Hydrogen from HCl can be prepared by :

Options

A. Cu
B. P
C. Mg
D. Hg

Explanation

To prepare hydrogen from hydrochloric acid (HCl), we need to react it with a suitable metal that can displace hydrogen from the acid. Let's analyze the options provided step by step.

### Step-by-Step Solution:

1. **Identify the Reactants**:
- We have hydrochloric acid (HCl) as our reactant. We need to find a metal that can react with HCl to produce hydrogen gas.

2. **Option A: Copper (Cu)**:
- Check the reactivity series: Hydrogen is above copper in the reactivity series, meaning hydrogen is more reactive than copper.
- Since copper cannot displace hydrogen from HCl, there will be no reaction.
- **Conclusion**: Copper cannot prepare hydrogen from HCl. (Incorrect option)

3. **Option B: Phosphorus (P)**:
- Phosphorus is a non-metal and does not participate in the reactivity series in the same way metals do.
- The reaction of phosphorus with HCl produces phosphorus trichloride (PCl3) and phosphine (PH3), but no hydrogen gas is produced.
- **Conclusion**: Phosphorus cannot prepare hydrogen from HCl. (Incorrect option)

4. **Option C: Magnesium (Mg)**:
- Magnesium is above hydrogen in the reactivity series, meaning it is more reactive than hydrogen.
- When magnesium reacts with HCl, it displaces hydrogen and forms magnesium chloride (MgCl2) and hydrogen gas (H2).
- The balanced reaction is:
\[ Mg + 2HCl \rightarrow MgCl_2 + H_2 \uparrow \]
- **Conclusion**: Magnesium can prepare hydrogen from HCl. (Correct option)

5. **Option D: Mercury (Hg)**:
- Mercury is below copper in the reactivity series, meaning it is less reactive than hydrogen.
- Therefore, mercury cannot displace hydrogen from HCl, and no reaction will occur.
- **Conclusion**: Mercury cannot prepare hydrogen from HCl. (Incorrect option)

### Final Answer:
The correct option for preparing hydrogen from HCl is **Option C: Magnesium (Mg)**.