Note : Consider the following four compounds for answering questions 19 and 20.
I. `CH_3 - CH_2 - CH_2 - overset(overset(O)||)C - H`
II. `CH_3 - CH_2 - CH_2 - overset(overset(O)||) C - CH_3`
III. `CH_3 - CH_2 - underset(underset(O)||) C - CH_2 - CH_3`
IV. `CH_3 - underset(underset(CH_3)|)CH - CH_2 - underset(underset(O)||)C - H`
Which of the following pairs are not functional group isomers?

Options

A. II and III
B. II and IV
C. I and IV
D. I and II

Explanation

To determine which pairs of the given compounds are not functional group isomers, we will analyze the structures of each compound and identify their functional groups.

### Step 1: Identify the compounds and their functional groups

1. **Compound I:** `CH3 - CH2 - CH2 - C(=O) - H`
- This is an aldehyde (specifically, butanal).

2. **Compound II:** `CH3 - CH2 - CH2 - C(=O) - CH3`
- This is a ketone (specifically, pentan-2-one).

3. **Compound III:** `CH3 - CH2 - C(=O) - CH2 - CH3`
- This is also a ketone (specifically, pentan-3-one).

4. **Compound IV:** `CH3 - C(CH3) - CH2 - C(=O) - H`
- This is an aldehyde (specifically, 3-methylbutanal).

### Step 2: Compare the functional groups

- **Compound I** and **Compound IV** both contain aldehyde functional groups.
- **Compound II** and **Compound III** both contain ketone functional groups.

### Step 3: Identify pairs that are not functional group isomers

- **Pair 1 (I and IV):** Both are aldehydes. **Not functional group isomers.**
- **Pair 2 (II and III):** Both are ketones. **Not functional group isomers.**
- **Pair 3 (I and II):** One is an aldehyde and the other is a ketone. **Functional group isomers.**
- **Pair 4 (I and III):** One is an aldehyde and the other is a ketone. **Functional group isomers.**
- **Pair 5 (II and IV):** One is a ketone and the other is an aldehyde. **Functional group isomers.**
- **Pair 6 (III and IV):** One is a ketone and the other is an aldehyde. **Functional group isomers.**

### Conclusion

The pairs that are not functional group isomers are:
- **Pair I and IV (Aldehyde and Aldehyde)**
- **Pair II and III (Ketone and Ketone)**

### Final Answer
The pairs that are not functional group isomers are **I and IV** and **II and III**.