Functional group isomerism refers to the phenomenon where compounds have the same molecular formula but different functional groups attached to the same carbon skeleton. In this question, we need to identify the compound in which functional group isomerism is not possible.

Option 1: Alcohols - Alcohols have the functional group -OH attached to the carbon skeleton. Functional group isomerism is possible in alcohols as we can have compounds with different functional groups like ethers or aldehydes.

Option 2: Aldehydes - Aldehydes have the functional group -CHO attached to the carbon skeleton. Functional group isomerism is possible in aldehydes as we can have compounds with different functional groups like ketones or carboxylic acids.

Option 3: Alkyl halides - Alkyl halides have halogen atoms (F, Cl, Br, or I) attached to the carbon skeleton. Functional group isomerism is not possible in alkyl halides as there is no other functional group to replace the halogen atom.

Option 4: Cyanides - Cyanides have the functional group -CN attached to the carbon skeleton. Functional group isomerism is possible in cyanides as we can have compounds with different functional groups like isoc