(A): Central atom usually forms three single and one double bonds in an oxyacid of phosphorus
(R): Phosphorus atom usually undergoes `sp^(3)` hybridisation in its oxyacids 

Options

A. Both A & R are true, R is the correct explanation of A
B. Both A & R are true, R is not correct explanation of A
C. A is true, R is false
D. A is false, R is true

Answer

To solve the question, we need to analyze both the assertion and the reason provided, and determine their validity and relationship.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the central atom in an oxyacid of phosphorus usually forms three single bonds and one double bond.
- In oxyacids of phosphorus, such as phosphoric acid (H₃PO₄), the phosphorus atom is the central atom.
- The structure of phosphoric acid can be represented as:
```
O
||
H-O-P-O-H
|
O-H
```
- In this structure, phosphorus forms three single bonds with three hydroxyl (–OH) groups and one double bond with one oxygen atom.

2. **Evaluating the Assertion**:
- The assertion is true because phosphorus does indeed form three single bonds and one double bond in its oxyacids.

3. **Understanding the Reason**:
- The reason states that the phosphorus atom usually undergoes sp³ hybridization in its oxyacid state.
- In the case of phosphoric acid, phosphorus has four regions of electron density (three single bonds and one double bond), which corresponds to sp³ hybridization.

4. **Evaluating the Reason**:
- The reason is also true because the hybridization of phosphorus in its oxyacid form is indeed sp³.

5. **Analyzing the Relationship**:
- While both the assertion and the reason are true, the reason does not directly explain the assertion. The formation of three single bonds and one double bond is a characteristic of the bonding in oxyacids, while sp³ hybridization describes the geometry and electron arrangement around the phosphorus atom.

6. **Conclusion**:
- Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion. Therefore, the correct answer is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- Both the assertion and reason are true, but the reason is not the correct explanation for the assertion.