Which of the following metals on reaction with sodium hydroxide solution produce hydrogen gas ?
1. Cu
2. Al
3. Fe
4. Zn

Options

A. 2 and 3
B. 2 and 4
C. 1 and 4
D. 2 only

Explanation

To determine which metals produce hydrogen gas when they react with sodium hydroxide (NaOH), we can analyze the reactions of each metal mentioned in the question: Copper (Cu), Aluminium (Al), Iron (Fe), and Zinc (Zn).

### Step-by-Step Solution:

1. **Reaction of Copper (Cu) with Sodium Hydroxide (NaOH)**:
- When copper reacts with sodium hydroxide, it forms copper(II) hydroxide (Cu(OH)₂).
- The reaction does not produce hydrogen gas.
- **Equation**:
\[ \text{Cu} + 2 \text{NaOH} \rightarrow \text{Cu(OH)}_2 + \text{Na}_2\text{O} \]
- **Conclusion**: Copper does not produce hydrogen gas.

2. **Reaction of Aluminium (Al) with Sodium Hydroxide (NaOH)**:
- Aluminium reacts vigorously with sodium hydroxide, producing sodium aluminate and hydrogen gas.
- **Equation**:
\[ 2 \text{Al} + 2 \text{NaOH} + 2 \text{H}_2\text{O} \rightarrow 2 \text{NaAlO}_2 + 3 \text{H}_2 \]
- **Conclusion**: Aluminium produces hydrogen gas.

3. **Reaction of Iron (Fe) with Sodium Hydroxide (NaOH)**:
- Iron reacts with sodium hydroxide to form iron(III) hydroxide, but it does not produce hydrogen gas.
- **Equation**:
\[ \text{Fe} + 3 \text{NaOH} \rightarrow \text{Fe(OH)}_3 + 3 \text{Na} \]
- **Conclusion**: Iron does not produce hydrogen gas.

4. **Reaction of Zinc (Zn) with Sodium Hydroxide (NaOH)**:
- Zinc reacts with sodium hydroxide to form sodium zincate and hydrogen gas.
- **Equation**:
\[ \text{Zn} + 2 \text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \]
- **Conclusion**: Zinc produces hydrogen gas.

### Final Answer:
From the analysis, the metals that produce hydrogen gas when reacting with sodium hydroxide are Aluminium (Al) and Zinc (Zn). Therefore, the correct options are:

- **B: 2 and 4 (Al and Zn)**.