**Structure of CH3COOH (Acetic Acid):**

The structure of acetic acid, CH3COOH, is composed of three main parts: a methyl group (CH3), a carbonyl group (C=O), and a hydroxyl group (OH).

- The methyl group (CH3) is a single carbon atom bonded to three hydrogen atoms. It is attached to the carbonyl group.
- The carbonyl group (C=O) is a carbon atom double-bonded to an oxygen atom. It is the functional group responsible for the acidity of acetic acid.
- The hydroxyl group (OH) is a hydrogen atom bonded to an oxygen atom. It is attached to the carbon atom of the carbonyl group.

The structural formula of acetic acid is as follows:

```
H
|
H - C - C = O
|
O - H
```

**Structure of CH3CH2COOH (Propanoic Acid):**

The structure of propanoic acid, CH3CH2COOH, is composed of four main parts: an ethyl group (CH3CH2), a carbonyl group (C=O), and a hydroxyl group (OH).

- The ethyl group (CH3CH2) is a two-carbon chain with three hydrogen atoms attached to the first carbon and one hydrogen atom attached to the second carbon. It is attached to the carbonyl group.
- The carbonyl group (C=O) is a carbon atom double-bonded to an oxygen atom. It is the functional group responsible for the acidity of propanoic acid.
- The hydroxyl group (OH) is a hydrogen atom bonded to an oxygen atom. It is attached to the carbon atom of the carbonyl group.

The structural formula of propanoic acid is as follows:

```
H
|
H - C - C - C = O
|
O - H
```

In both acetic acid and propanoic acid, the presence of the carbonyl group (C=O) and the hydroxyl group (OH) contribute to their acidity. The hydrogen atom in the hydroxyl group is easily released, resulting in the formation of H+ ions in an aqueous solution, making these compounds acidic. The difference in the number of carbon atoms between the two compounds (one in acetic acid and two in propanoic acid) affects their physical and chemical properties, such as boiling points, solubilities, and reactivity.

D option is right.the alcohol pentan2 ol undergo dehydration first and forms pent2 ene(CH3-CH=CH-CH2-CH3).this pent 2-en undergoes oxidation and form CH3-COOH &CH3-CH2-COOH by Popoff's rule.