A gaseous substance dissolve in water giving a pale blue solution which decolouises `KMnO_(4)` and oxidises `Kl` to `l_(2)` in acidic medium :

Options

A. `N_(2)O_(5)`
B. `NH_(3)`
C. `N_(2)O_(3)`
D. `HNO_(3)`

Explanation

To solve the problem, we need to identify the gaseous substance that dissolves in water to form a pale blue solution, which can decolorize KMnO4 and oxidize KI to I2 in an acidic medium. Let's break down the steps to find the correct answer.

### Step-by-Step Solution:

1. **Understanding the Problem:**
- We are looking for a gaseous substance that, when dissolved in water, creates a pale blue solution.
- This solution must be able to decolorize potassium permanganate (KMnO4) and oxidize potassium iodide (KI) to iodine (I2) in an acidic medium.

2. **Analyzing the Options:**
- **Option A: N2O5**
- N2O5 is a nitrogen oxide that forms nitric acid when dissolved in water. It does not create a pale blue solution.
- **Option B: NH3**
- Ammonia (NH3) does not produce a pale blue solution and does not have the oxidizing properties required for this reaction.
- **Option C: N2O3**
- Dinitrogen trioxide (N2O3) reacts with water to form nitrous acid (HNO2), which is known to have a pale blue color.
- **Option D: HNO3**
- Nitric acid (HNO3) is a strong oxidizer but does not produce a pale blue solution.

3. **Identifying the Reaction:**
- When N2O3 dissolves in water, it forms HNO2:
\[ N2O3 + H2O \rightarrow 2HNO2 \]
- HNO2 can decolorize KMnO4 (which is purple) in acidic conditions:
\[ 5HNO2 + 2KMnO4 + 6HCl \rightarrow 2MnCl2 + 2KCl + 5HNO3 + 3H2O \]
- HNO2 also oxidizes iodide ions (from KI) to iodine:
\[ 2HNO2 + 6KI + 6HCl \rightarrow N2 + 3I2 + 6KCl + 3H2O \]

4. **Conclusion:**
- Based on the analysis, the gaseous substance that meets all the criteria is **N2O3**. It dissolves in water to form a pale blue solution (HNO2), which can decolorize KMnO4 and oxidize KI to I2.

### Final Answer:
The correct option is **C: N2O3**.