Assertion: Acetaldehyde on treatment with alkali gives aldol.
Reason: Acetaldehyde molecules contains `alpha` hydrogen atom.

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion
B. If both assertion and reason are true but reason is not the correct explanation of the assertion
C. If assertion is true but reason is false
D. If the assertion and reason both are false

Explanation

To solve the question regarding the assertion and reason about acetaldehyde and its reaction with alkali, we can break it down into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that acetaldehyde, when treated with an alkali, gives an aldol.
- Aldol refers to a β-hydroxy aldehyde or ketone that is formed through the aldol condensation reaction.

2. **Identifying Acetaldehyde Structure**:
- Acetaldehyde has the chemical formula CH₃CHO.
- Its structure can be represented as:
```
H
|
C=O
|
H
|
CH₃
```

3. **Presence of Alpha Hydrogen**:
- The reason states that acetaldehyde contains alpha hydrogen atoms.
- In acetaldehyde, the carbon adjacent to the carbonyl group (C=O) has hydrogen atoms attached (the methyl group CH₃ provides these alpha hydrogens).

4. **Mechanism of Aldol Reaction**:
- When acetaldehyde is treated with a base (alkali), the alpha hydrogen is abstracted by the hydroxide ion (OH⁻) from the base, forming an enolate ion.
- The enolate ion then acts as a nucleophile and attacks the carbonyl carbon of another acetaldehyde molecule.

5. **Formation of Aldol Product**:
- The nucleophilic addition results in the formation of a β-hydroxy aldehyde (aldol).
- The reaction can be summarized as:
- Enolate ion + Acetaldehyde → β-hydroxybutyraldehyde (aldol)
- The aldol can further dehydrate to form an α,β-unsaturated aldehyde.

6. **Conclusion**:
- Both the assertion and reason are correct. The presence of alpha hydrogen in acetaldehyde allows it to undergo the aldol reaction when treated with alkali.

### Final Answer:
Both assertion and reason are true, and the reason is the correct explanation of the assertion.