The chain isomer of butane is:
Chain, nuclear or skeleton isomerism: This type of isomerism is arising due to the difference in the nature of the carbon chain (i.e., straight or branched) which forms the nucleus of the molecule.
So, Butane is CH3−CH2−CH2−CH3
Option A and B are wrong because they have 5 carbons. In isomers, the molecular formula does not change.
In option (C), The parent carbon chain is different. But the molecular formula is the same.