A : `H_(3) PO_4` is less acidic than `H_(3) PO_(3)` .
R : Oxidation state of phosphorus in `H_(3) PO_(4) lt H_(3) PO_(3)` .

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 question, we need to analyze the assertion and reasoning provided regarding the acidity of phosphoric acid (H₃PO₄) and phosphorous acid (H₃PO₃), as well as the oxidation states of phosphorus in these compounds.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that H₃PO₄ is less acidic than H₃PO₃.
- Acidity in these compounds can be assessed by looking at their ability to donate protons (H⁺ ions).

2. **Drawing the Structures**:
- Draw the Lewis structures of both acids:
- H₃PO₄: Phosphorus is bonded to four oxygen atoms, one of which is an OH group (hydroxyl).
- H₃PO₃: Phosphorus is bonded to three oxygen atoms, one of which is also an OH group, and one of the oxygen atoms is a double bond.

3. **Formation of Conjugate Bases**:
- When H₃PO₄ loses a proton, it forms H₂PO₄⁻.
- When H₃PO₃ loses a proton, it forms H₂PO₃⁻.
- Analyze the stability of these conjugate bases. The more stable the conjugate base, the stronger the acid.

4. **Resonance Structures**:
- Both conjugate bases can exhibit resonance, which stabilizes them.
- H₂PO₄⁻ has more resonance structures compared to H₂PO₃⁻ due to the presence of more oxygen atoms that can participate in resonance.

5. **Effect of Hydroxyl Group**:
- In H₃PO₄, the presence of two hydroxyl groups can disrupt the resonance stabilization compared to H₃PO₃, which has only one.
- This disruption makes H₃PO₄ less acidic than H₃PO₃.

6. **Conclusion on Assertion**:
- Since H₃PO₄ is indeed less acidic than H₃PO₃, the assertion is **true**.

7. **Understanding the Reasoning**:
- The reasoning states that the oxidation state of phosphorus in H₃PO₄ is less than in H₃PO₃.
- To find the oxidation states:
- For H₃PO₄: Let the oxidation state of phosphorus be X.
- 3(1) + X + 4(-2) = 0 → 3 + X - 8 = 0 → X = +5.
- For H₃PO₃: Let the oxidation state of phosphorus be Y.
- 3(1) + Y + 3(-2) = 0 → 3 + Y - 6 = 0 → Y = +3.

8. **Conclusion on Reasoning**:
- The oxidation state of phosphorus in H₃PO₄ is +5, which is **greater** than the oxidation state of +3 in H₃PO₃.
- Therefore, the reasoning is **false**.

9. **Final Answer**:
- The assertion is true, but the reasoning is false.

### Summary:
- **Assertion**: True (H₃PO₄ is less acidic than H₃PO₃).
- **Reasoning**: False (Oxidation state of phosphorus in H₃PO₄ is greater than in H₃PO₃).