Assertion : Buna-S is a copolymer .
Reason : Buna-S is formed by condensation reaction between two different monomers.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion .
B. If both assertion and reason are true but reason is not the correct explanation of assertion .
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

### Step-by-Step Solution

1. **Understanding the Assertion**:
- The assertion states that "Buna-S is a copolymer."
- A copolymer is defined as a polymer that is made from two or more different types of monomer units.

2. **Identifying the Components of Buna-S**:
- Buna-S is made from two monomers: Buta-1,3-diene and Styrene.
- Buta-1,3-diene has the structure: CH2=CH-CH=CH2.
- Styrene has the structure: C6H5-CH=CH2.

3. **Polymerization Process**:
- The polymerization of Buna-S occurs through a process known as addition polymerization.
- In this process, the double bonds in the monomers open up and link together to form a long chain polymer.

4. **Evaluating the Reason**:
- The reason provided states that "Buna-S is formed by a condensation reaction between two different monomers."
- This statement is incorrect because Buna-S is formed through addition polymerization, not condensation polymerization.

5. **Conclusion**:
- The assertion is true (Buna-S is a copolymer).
- The reason is false (Buna-S is not formed by a condensation reaction).
- Therefore, the correct answer is that the assertion is true, but the reason is false.

### Final Answer:
- **Assertion**: True
- **Reason**: False
- **Correct Option**: Assertion is true, but the reason is false (Option C).