In which of the following, functional group isomerism is not possible?

Two (or) more compounds having the same molecular formula but different  functional groups are called functional isomers.Functional isomer of alcohol is etherFunctional isomer of CH3 CH2 OH is  CH3OCH3Functional isomer of aldehyde is ketoneFunctional isomer of  CH3 CH2 CHO is  CH3COCH3Functional isomer of cyanides is isocyanidesFunctional isomer of  CH3 CN is  CH3NCHowever, alkylhalides do not show functional isomerism.Hence, option (4) CH3 CH2 Cl  is correct