Assertion: The `pK_(a)` of acetic acid is lower than that of phenol.
Reason : carboxylate ion is more resonance sabilised.

Options

A. If both assertion and reason are correct and reason is correct explanation for assertion.
B. If both assertion and reason are correct but reason is not correct for assertion.
C. If assertion is correct but reason is incorrect.
D. If both assertion and reason are incorrect.

Explanation

To solve the question regarding the assertion and reason about the acidity of acetic acid compared to phenol, let's break it down step by step.

### Step 1: Understand the Assertion
The assertion states that the pKa of acetic acid is lower than that of phenol.

- **Explanation**: The pKa value is a measure of the acidity of a compound; a lower pKa value indicates a stronger acid. Acetic acid (CH₃COOH) is indeed a stronger acid than phenol (C₆H₅OH), which means it has a lower pKa value.

### Step 2: Understand the Reason
The reason given is that the carboxylate ion (the conjugate base of acetic acid) is more resonance stabilized than the phenoxide ion (the conjugate base of phenol).

- **Explanation**: When acetic acid donates a proton (H⁺), it forms the carboxylate ion (CH₃COO⁻). This ion is stabilized by resonance, as the negative charge can be delocalized over two oxygen atoms. In contrast, when phenol donates a proton, it forms the phenoxide ion (C₆H₅O⁻), which is also resonance stabilized but has fewer resonance structures compared to the carboxylate ion.

### Step 3: Compare the Stability of Conjugate Bases
- **Carboxylate Ion**: The carboxylate ion has two equivalent resonance structures, which allows for better delocalization of the negative charge.
- **Phenoxide Ion**: The phenoxide ion has fewer resonance structures, which means the negative charge is less delocalized compared to the carboxylate ion.

### Step 4: Conclusion
Since the carboxylate ion is more resonance stabilized than the phenoxide ion, it follows that acetic acid is more acidic than phenol. Therefore, both the assertion and the reason are correct, and the reason correctly explains the assertion.

### Final Answer
Both the assertion and the reason are correct, and the reason is the correct explanation for the assertion.