Assertion: Red phophorus is less volatile then white phosphorus.
Reason: Red phosphorus has a discrete tetrahedral structure.

Options

A. If both (A) and (R) are true and (R) is the correct explanation of (A)
B. if both (A) and (R) are true but (R) is not correct explanation of (A)
C. If (A) is true but (R) is false
D. If (A) is false but (R) is false

Explanation

To solve the assertion and reason question regarding red phosphorus and white phosphorus, we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- The assertion states that "Red phosphorus is less volatile than white phosphorus."
- Volatility refers to how readily a substance vaporizes. Generally, substances with weaker intermolecular forces are more volatile.

2. **Analyzing the Structures:**
- **White Phosphorus (P4):**
- White phosphorus consists of discrete P4 tetrahedral molecules.
- The molecules are held together by weak Van der Waals forces, which makes them more volatile.
- **Red Phosphorus:**
- Red phosphorus has a polymeric structure where phosphorus atoms are linked together in chains.
- These chains are held together by stronger covalent bonds, making red phosphorus less volatile compared to white phosphorus.

3. **Conclusion on the Assertion:**
- Since red phosphorus has stronger covalent bonds in its structure compared to the weak Van der Waals forces in white phosphorus, the assertion that "red phosphorus is less volatile than white phosphorus" is **true**.

4. **Understanding the Reason:**
- The reason states that "Red phosphorus has a discrete tetrahedral structure."
- However, this statement is incorrect because it is white phosphorus that has a discrete tetrahedral structure, while red phosphorus has a polymeric structure.

5. **Conclusion on the Reason:**
- The reason provided is **false** because it incorrectly describes the structure of red phosphorus.

6. **Final Evaluation:**
- The assertion is true, and the reason is false. Therefore, the correct option is that the assertion is true but the reason is false.

### Final Answer:
The correct option is **Option 3: A is true but reason is false.**