A : HCOOH has less `pK_(a)` than `C_(6)H_(5)COOH`.
R : In `C_(6)H_(5)COOH,C_(6)H_(5)`- group shows - l effect not +R effect

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To solve the problem, we need to analyze the statements given:

**A:** HCOOH has less pKa than C6H5COOH.
**R:** In C6H5COOH, the C6H5 group shows -I effect, not +R effect.

### Step-by-Step Solution:

1. **Understanding pKa Values:**
- pKa is a measure of the strength of an acid. A lower pKa value indicates a stronger acid.
- Formic acid (HCOOH) and benzoic acid (C6H5COOH) are the acids we are comparing.

2. **Comparing Acid Strength:**
- To determine which acid is stronger, we need to look at the stability of the conjugate base formed after the acid donates a proton (H+).
- For HCOOH, the conjugate base is HCOO⁻ (formate ion).
- For C6H5COOH, the conjugate base is C6H5COO⁻ (benzoate ion).

3. **Stability of Conjugate Bases:**
- The stability of the conjugate base affects the acidity of the acid.
- The formate ion (HCOO⁻) is stabilized by resonance between the two oxygen atoms.
- The benzoate ion (C6H5COO⁻) also has resonance stabilization, but the presence of the phenyl group (C6H5) introduces additional factors.

4. **Effect of the Phenyl Group:**
- The phenyl group can exhibit resonance (R effect) which can stabilize the negative charge on the benzoate ion.
- However, the phenyl group also has a -I (inductive) effect, which can withdraw electron density from the carboxylate group, reducing the overall stability of the conjugate base.

5. **Conclusion on Acidity:**
- Since the formate ion is more stable than the benzoate ion due to the effective resonance stabilization without the destabilizing -I effect, HCOOH is a stronger acid than C6H5COOH.
- Therefore, A is true: HCOOH has less pKa than C6H5COOH.

6. **Analyzing the Reason:**
- The reason states that in C6H5COOH, the C6H5 group shows -I effect, not +R effect.
- This statement is misleading. The C6H5 group does show +R effect due to resonance, which helps stabilize the negative charge on the benzoate ion.
- Therefore, R is false.

### Final Evaluation:
- Statement A is true, and statement R is false.