Functional isomer is not possible for -

1. Alcohols 2. Aldehydes
3. Alkyl halides 4. Cyanides
Hint: Halogen containing compound didnot show functional group isomerism
Functional isomerism occurs when substances have the same molecular formula but different functional groups. 
Functional isomer of alcohol is ether.
Functional isomer of aldehyde is ketone.
Funtional isomer of aldehyde is ketone.
Functional isomer of cyanide is isocyanide
However, alkyl halides do not show functional isomerism. Hence, option (3) is correct.