The carbonyl compounds listed are 2-pentanone, 3-pentanone, 3-pentenal, and 3-hexanone. We need to determine which of these compounds have four enol isomers.

Enol isomers are tautomeric forms of carbonyl compounds, in which the hydrogen atom on the α-carbon is replaced by a double bond with the oxygen atom. To determine the number of enol isomers, we need to look at the possible positions for the double bond in the molecule.

1. 2-pentanone:
In 2-pentanone, there is no α-carbon adjacent to the carbonyl carbon, so it cannot form an enol isomer. Therefore, it does not have any enol isomers.

2. 3-pentanone:
In 3-pentanone, there is an α-carbon adjacent to the carbonyl carbon, and it can form an enol isomer. The double bond can be formed between the α-carbon and the oxygen atom. Thus, 3-pentanone has one enol isomer.

3. 3-pentenal:
In 3-pentenal, there is also an α-carbon adjacent to the carbonyl carbon. The double bond can be formed between the α-carbon and the oxygen atom, similar to 3-pentanone. However, in addition to this enol isomer, 3-pentenal can also form a double bond between the α-carbon and the carbon atom in the aldehyde group. This results in two enol isomers for 3-pentenal.

4. 3-hexanone:
In 3-hexanone, there is an α-carbon adjacent to the carbonyl carbon, and it can form an enol isomer. The double bond can be formed between the α-carbon and the oxygen atom. Additionally, there is another α-carbon adjacent to the carbonyl carbon, which can also form a double bond with the oxygen atom. Therefore, 3-hexanone has two enol isomers.

In summary, the carbonyl compounds with four enol isomers are 3-pentenal and 3-hexanone (options C and D). These compounds have multiple α-carbons adjacent to the carbonyl carbon, allowing for the formation of different enol isomers.