Assertion : But-1-ene and 2-methylprop-1-ene are position isomers.
Reason : Position isomers have same molecular formula but different arrangement of carbon atoms.

Options

A. If both assertion and reason are correct and reason is correct explanation for assertion.
B. If both assertion and reason are correct but reason is not correct explanation for assertion.
C. If assertion is correct but reason is incorrect.
D. If both assertion and reason are incorrect.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that but-1-ene and 2-methylprop-1-ene are position isomers.
- **But-1-ene** has the structure: CH2=CH-CH2-CH3.
- **2-Methylprop-1-ene** has the structure: CH2=C(CH3)-CH2-CH3.

2. **Identifying the Type of Isomers**:
- Position isomers are compounds that have the same molecular formula but differ in the position of a functional group or a double bond.
- In this case, both compounds have the same molecular formula (C4H8), but we need to check the position of the double bond.

3. **Analyzing the Double Bond**:
- In but-1-ene, the double bond is between the first and second carbon atoms.
- In 2-methylprop-1-ene, the double bond is also between the first and second carbon atoms (when considering the carbon chain).
- Thus, the position of the double bond remains the same in both compounds.

4. **Conclusion on Assertion**:
- Since the position of the double bond does not differ, the assertion that they are position isomers is **false**.

5. **Understanding the Reason**:
- The reason states that position isomers have the same molecular formula but different arrangements of carbon atoms.
- While it is true that position isomers have the same molecular formula, the arrangement of carbon atoms must also include the position of the double bond or functional group.

6. **Conclusion on Reason**:
- The reason is also **incorrect** because it does not accurately describe the nature of position isomers.

7. **Final Conclusion**:
- Both the assertion and reason are incorrect. Therefore, the correct answer is that both the assertion and reason are false.

### Final Answer:
Both the assertion and reason are incorrect.