Assertion (A): Pentane and `3` methyl pentane are chain isomers.
Reason (R ): Pentane is a straight-chain alkane while `3`-methyl pentane is branched-chain alkane.

Options

A. both (A) and (R ) are true and (R ) is the correct explanation for (A).
B. both (A) and (R ) are true and (R ) is not the true explanation for (A)
C. (A) is true, but (R ) is false.
D. (A) is false and (R ) is true.

Explanation

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

**Step 1: Understand the Assertion (A)**
- The assertion states that pentane and 3-methylpentane are chain isomers.
- Chain isomers are compounds that have the same molecular formula but different arrangements of the carbon skeleton (i.e., different types of carbon chains).

**Step 2: Identify the Structures**
- Pentane (C5H12) is a straight-chain alkane, represented as:
```
CH3-CH2-CH2-CH2-CH3
```
- 3-methylpentane (C6H14) has the structure:
```
CH3
|
CH3-CH2-CH-CH2-CH3
```
Here, a methyl group (CH3) is attached to the third carbon of the pentane chain.

**Step 3: Analyze the Structures**
- Both pentane and 3-methylpentane have different structures. Pentane is a straight-chain alkane, while 3-methylpentane is a branched-chain alkane.
- However, they do not have the same molecular formula. Pentane has the formula C5H12, while 3-methylpentane has the formula C6H14. Therefore, they cannot be chain isomers.

**Step 4: Understand the Reason (R)**
- The reason states that pentane is a straight-chain alkane and 3-methylpentane is a branched-chain alkane.
- This statement is indeed correct. Pentane is a straight-chain alkane, and 3-methylpentane is a branched-chain alkane.

**Step 5: Conclusion**
- Since the assertion (A) is false (they are not chain isomers), and the reason (R) is true (the description of the structures is correct), we conclude:
- Assertion (A): False
- Reason (R): True

Thus, the correct answer is that A is false and R is true.