`M("Salt")+"Dil. "HCl overset(Warm)to P+Q uarr+H_(2)O`
gas 'Q' changes colour of `FeSO_(4)` solution into yellow solution then salt M in above reaction is

Options

A. `BaS_(2)O_(3)`
B. `Ag_(2)SO_(3)`
C. `AgNO_(2)`
D. `Pb(NO_(3))_(2)`

Explanation

To solve the question step by step, we need to identify the salt \( M \) that reacts with dilute hydrochloric acid (HCl) to produce a gas \( Q \) which changes the color of an iron(II) sulfate (FeSO4) solution to yellow.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The reaction involves a salt \( M \) reacting with dilute HCl upon warming to produce products \( P \), gas \( Q \), and water (H2O). The key detail is that gas \( Q \) changes the color of FeSO4 solution to yellow.

2. **Identifying the Color Change**:
The yellow color change in the FeSO4 solution suggests the formation of iron(III) ions (Fe³⁺), which typically occurs when a reducing agent is present. Common gases that can reduce Fe²⁺ to Fe³⁺ include sulfur dioxide (SO2) and nitrogen dioxide (NO2).

3. **Evaluating the Given Options**:
- **Option 1: BaS2O3**
- Reaction: \( BaS2O3 + 2HCl \rightarrow BaCl2 + SO2 + H2O \)
- Gas \( Q \) is SO2, which does not change the color of FeSO4 to yellow. **Incorrect.**

- **Option 2: Ag2SO3**
- Reaction: \( Ag2SO3 + 2HCl \rightarrow 2AgCl + SO2 + H2O \)
- Again, gas \( Q \) is SO2, which does not change the color of FeSO4 to yellow. **Incorrect.**

- **Option 3: AgNO2**
- Reaction: \( AgNO2 + HCl \rightarrow AgCl + NO2 + H2O \)
- Here, gas \( Q \) is NO2, which can oxidize Fe²⁺ to Fe³⁺, resulting in a yellow solution. **Correct.**

- **Option 4: Pb(NO3)2**
- This salt does not decompose with dilute HCl. **Incorrect.**

4. **Conclusion**:
The correct salt \( M \) that reacts with dilute HCl to produce a gas \( Q \) that changes the color of FeSO4 solution to yellow is **AgNO2**.

### Final Answer:
The salt \( M \) in the above reaction is **AgNO2**.