Mustard gas is formed when gas react with `S_2Cl_2`. The gas is

Options

A. ethane
B. butane
C. propene
D. ethylene

Explanation

To determine which gas reacts with \( S_2Cl_2 \) to form mustard gas, we need to analyze the reaction between \( S_2Cl_2 \) and the given options: butane, propane, and ethylene.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- We have \( S_2Cl_2 \) (disulfur dichloride) and three potential gases: butane (\( C_4H_{10} \)), propane (\( C_3H_8 \)), and ethylene (\( C_2H_4 \)).

**Hint**: Understand the structure and properties of each compound. Ethylene is an alkene, while butane and propane are alkanes.

2. **Recognize the Reaction Type**:
- Mustard gas is a type of chemical compound known as a sulfur mustard. It is typically formed through a reaction involving alkenes and sulfur-containing compounds.

**Hint**: Recall that alkenes can undergo addition reactions due to their double bonds.

3. **Analyze Ethylene**:
- Ethylene (\( C_2H_4 \)) has a double bond, which makes it reactive. When ethylene reacts with \( S_2Cl_2 \), it can undergo a reaction where the double bond opens up to form a new compound.

**Hint**: Look for the presence of double bonds in the gas options, as they are more likely to react with \( S_2Cl_2 \).

4. **Write the Reaction**:
- The reaction of ethylene with \( S_2Cl_2 \) can be represented as:
\[ C_2H_4 + S_2Cl_2 \rightarrow \text{Mustard Gas} \]
- The product formed is mustard gas, which has the structure \( (ClCH_2)_2S \).

**Hint**: Consider the products formed in reactions involving sulfur and chlorine.

5. **Conclusion**:
- Based on the analysis, the gas that reacts with \( S_2Cl_2 \) to form mustard gas is ethylene.

**Final Answer**: The gas that reacts with \( S_2Cl_2 \) to form mustard gas is **ethylene**.