(C) (ii) and (iii)
Structures (ii) and (iii) both have the molecular formula C₄H₁₀ (butane) but different carbon skeletons — they are structural isomers.
Structural isomers have the same molecular formula but different structures. (ii) is n-butane (straight chain) and (iii) is isobutane (branched chain) — both C₄H₁₀. Structure (i) is but-1-ene (C₄H₈, unsaturated), and (iv) has a Cl substituent, making it a haloalkane with a different molecular formula. Examiners expect you to identify identical molecular formula + different structure = structural isomers.