Among these III is the chain isomer of

Isomers have the same number of atoms or the same molecular formula but the structural formulas are different for an isomer. A chain isomer is the one which has the straight chain branched. This means that one or two of the groups will be branched with the main chain. Here, we can see that in structure I the -OH group is present in the main chain while in structure II it is branched with the second carbon. Structure III has a methyl group and the –OH group both are branched with the second carbon. Since all of them have the same molecular formula, they are isomers and because of the branching