The correct option is A Chain isomers
Pentanal and 2-Methyl butanal are chain isomers.
Chain isomerism is due to the difference in the nature of the carbon chain (i.e, straight or branched) which forms the nucleus of the molecule. Pentanal is CH3CH2CH2CH2CHO and 2-methyl butanal is
CH3CH2CH(CH3)CHO