Which one of the following hydrocarbon does not show positional isomerism and has three chain isomers ?

Alkanes show only one type of structural isomerism which is chain isomerism ;Number of structural isomers (all chain isomers)  :C4H10 — 2  ; C5H12 — 3  ; C6H14 — 5  ; C7H16 — 9  ; C8H18 — 18  ; C9H20 — 35  ; C10H22 — 75  ;C5H12 ---- n-pentane , isopentane and neopentane ;