A reagent is added to a solution of manganous salt in cold dil. `HNO_(3)`. A purple colour appeared due to the formation of permanganic acid. The reagent is:

Options

A. `NaBiO_(3)`
B. `(NH_(4))_(2)C_(2)O_(4)`
C. `H_(2)O_(2)`
D. `KNO_(3)`

Explanation

To solve the question regarding the reagent that leads to the formation of permanganic acid (HMnO4) from a solution of manganous salt (MnSO4) in cold dilute HNO3, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Manganous Salt**:
The manganous salt mentioned in the question is MnSO4 (manganese(II) sulfate).

2. **Understand the Reaction with Dilute HNO3**:
When MnSO4 is treated with cold dilute nitric acid (HNO3), it does not directly produce permanganic acid. Instead, we need a strong oxidizing agent to oxidize Mn²⁺ ions to MnO4⁻ ions.

3. **Identify the Reagent**:
The reagent that can oxidize Mn²⁺ to MnO4⁻ in acidic medium is sodium bismuthate (NaBiO3). This reagent is known for its strong oxidizing properties.

4. **Write the Balanced Chemical Equation**:
The reaction can be represented as:
\[ 2 \text{MnSO}_4 + 16 \text{HNO}_3 + 5 \text{NaBiO}_3 \rightarrow 2 \text{HMnO}_4 + 5 \text{Bi(NO}_3)_3 + 2 \text{Na}_2\text{SO}_4 + 7 \text{H}_2\text{O} \]
This equation shows that permanganic acid (HMnO4) is formed along with bismuth nitrate, sodium sulfate, and water.

5. **Conclusion**:
The reagent that causes the appearance of purple color due to the formation of permanganic acid is sodium bismuthate (NaBiO3).

### Final Answer:
The reagent is **NaBiO3** (sodium bismuthate).