The correct answer is D. Cyanides.
Functional group isomerism is a type of isomerism in which two or more compounds have the same molecular formula but different functional groups. In cyanides, the functional group is the cyanide group (CN). The cyanide group is always present in cyanides, so functional group isomerism is not possible.
Alcohols have the functional group hydroxyl (-OH). Aldehydes have the functional group aldehyde (-CHO). Alkyl halides have the functional group alkyl halide (R-X). All of these functional groups can be present in different compounds, so functional group isomerism is possible in these compounds.
For example, ethanol (CH3CH2OH) and 1-propanol (CH3CH2CH2OH) are both alcohols. They have the same molecular formula (C2H6O), but they have different functional groups. Ethanol has the hydroxyl group, while 1-propanol has the propyl group.
Acetaldehyde (CH3CHO) and formaldehyde (CH2O) are both aldehydes. They have the same molecular formula (CH3O), but they have different functional groups. Acetaldehyde has the aldehyde group, while formaldehyde has the methyl group.
Chloromethane (CH3Cl) and bromomethane (CH3Br) are both alkyl halides. They have the same molecular formula (CH3X), but they have different functional groups. Chloromethane has the chloro group, while bromomethane has the bromo group.