(C) (i) and (ii)
Structural isomers are compounds having the same molecular formula but different structures. The molecular formula of butane is C4H10. Structure (i) and (ii) have same molecular formula - C4H10, only the structures are different. Hence, they are structural isomers of butane. Structure (iii) and (iv) have molecular formula - C4H8 and hence, they are not structural isomers of butane.