I and III,

I and IV,

II and III,

II and IV,

II and IV,

V and VII,

VI and VII

Explanation:

Compounds having same molecular formula but vary in the length of the parent chain(as they are branched) is called chain isomerism. Compound I and II are linear (no branching) while III and IV are branched, so I and II are chain isomers with III and IV. Similarly, VII is ether with a branched alkyl group while V and VI are linear. So, VII is an isomer of both V and VI.