The gas which turns red litmus blue and is highly soluble in water is

Options

A. `NH_3`
B. `Cl_2`
C. `SO_2`
D. `H_2`

Explanation

To determine the gas that turns red litmus blue and is highly soluble in water, we can analyze each option provided:

1. **Option A: Ammonia (NH3)**
- Ammonia is a basic gas. When dissolved in water, it forms ammonium hydroxide (NH4OH), which is a base.
- Bases turn red litmus paper blue. Therefore, ammonia will turn red litmus blue.
- Ammonia is also highly soluble in water.

2. **Option B: Chlorine (Cl2)**
- Chlorine is a gas that is slightly soluble in water but is not a base.
- It does not turn red litmus blue; in fact, it can bleach litmus paper.

3. **Option C: Sulfur Dioxide (SO2)**
- Sulfur dioxide is an acidic gas. When it dissolves in water, it forms sulfurous acid (H2SO3).
- Acids turn blue litmus paper red, not the other way around.

4. **Option D: Hydrogen (H2)**
- Hydrogen gas does not react with water to form any acidic or basic solution.
- Therefore, it does not affect litmus paper.

Based on this analysis, the gas that turns red litmus blue and is highly soluble in water is **Option A: Ammonia**.

### Step-by-Step Solution:
1. **Identify the properties of each gas**: Analyze the behavior of each gas with respect to litmus paper and water solubility.
2. **Evaluate Ammonia**: Recognize that ammonia is a base and reacts with water to form ammonium hydroxide, which turns red litmus blue.
3. **Evaluate Chlorine**: Note that chlorine is slightly soluble and does not turn red litmus blue.
4. **Evaluate Sulfur Dioxide**: Understand that sulfur dioxide is acidic and would turn blue litmus red.
5. **Evaluate Hydrogen**: Acknowledge that hydrogen does not react with water and does not affect litmus paper.
6. **Conclude**: Based on the evaluations, conclude that ammonia is the correct answer.