Metal A' reacts with dil. `HNO_(3)` and liberates hydrogen. A is

Options

A. Mg
B. Ag
C. Cu
D. Au

Explanation

To solve the problem, we need to identify the metal A that reacts with dilute HNO3 and liberates hydrogen gas (H2).

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- When metals react with dilute nitric acid (HNO3), they can either produce hydrogen gas or form other products depending on their reactivity.
- The general reaction for a metal (M) with dilute HNO3 can be represented as:
\[ M + HNO3 \rightarrow M(NO3) + H2 \]

2. **Identifying Reactivity**:
- Not all metals can liberate hydrogen gas when reacting with dilute nitric acid.
- Metals that are higher in the reactivity series can react with dilute acids to produce hydrogen gas, while less reactive metals do not.

3. **Reactivity Series**:
- In the reactivity series, metals like magnesium (Mg) and manganese (Mn) are highly reactive and can react with dilute HNO3 to produce hydrogen.
- On the other hand, metals like copper (Cu), silver (Ag), and gold (Au) are much less reactive and do not produce hydrogen when reacting with dilute HNO3.

4. **Analyzing the Options**:
- The question provides the following options: magnesium, gold, silver, copper.
- Since we are looking for a metal that liberates hydrogen gas when reacting with dilute HNO3, we can eliminate gold, silver, and copper as they do not react in this manner.

5. **Conclusion**:
- The only metal from the options that can react with dilute HNO3 and liberate hydrogen gas is magnesium (Mg).
- Therefore, the answer is:
\[ \text{Metal A is Magnesium (Mg)} \]