The compound which forms only acetaldehyde upon ozonolysis is :

Options

A. Ethene
B. Propene
C. But-1-ene
D. But-2-ene

Explanation

To solve the question, "The compound which forms only acetaldehyde upon ozonolysis is:", we need to analyze the options given and determine which compound, when subjected to ozonolysis, will yield only acetaldehyde (CH3CHO).

### Step-by-Step Solution:

**Step 1: Understand Ozonolysis**
- Ozonolysis is a reaction where ozone (O3) reacts with alkenes or alkynes to form carbonyl compounds (aldehydes or ketones). The double bond in the hydrocarbon is cleaved, and ozone is inserted, leading to the formation of carbonyl groups.

**Step 2: Identify Acetaldehyde**
- Acetaldehyde has the structure CH3CHO. This means we need to find a compound that, upon ozonolysis, will only produce this compound.

**Step 3: Analyze Each Option**
1. **Ethene (C2H4)**:
- Ozonolysis of ethene:
- C2H4 → O3 → CH2O + CH2O (two molecules of formaldehyde, not acetaldehyde).
- **Conclusion**: Ethene does not yield only acetaldehyde.

2. **Propene (C3H6)**:
- Ozonolysis of propene:
- CH3-CH=CH2 → O3 → CH3CHO + CH2O (one molecule of acetaldehyde and one of formaldehyde).
- **Conclusion**: Propene does not yield only acetaldehyde.

3. **But-1-ene (C4H8)**:
- Ozonolysis of but-1-ene:
- CH2=CH-CH2-CH3 → O3 → CH3CHO + CH3CHO (two molecules of acetaldehyde).
- **Conclusion**: But-1-ene yields only acetaldehyde.

4. **But-2-ene (C4H8)**:
- Ozonolysis of but-2-ene:
- CH3-CH=CH-CH3 → O3 → CH3CHO + CH3CHO (two molecules of acetaldehyde).
- **Conclusion**: But-2-ene also yields only acetaldehyde.

**Step 4: Final Conclusion**
- Both but-1-ene and but-2-ene yield only acetaldehyde upon ozonolysis. However, the question asks for a compound that forms only acetaldehyde. The best answer is **But-1-ene** as it is a straight-chain alkene.

### Final Answer:
The compound which forms only acetaldehyde upon ozonolysis is **But-1-ene**.