Which pair of isomers given below are position isomers?

Position isomers share the same molecular formula but different arrangements of the position of functional groups.
For (A), Propanal and propanone do not share the same molecular formula. 
For (B), they share the same molecular formula i.e. ${\mathrm{C}}_4{\mathrm{H}}_{10}\mathrm{O}$, but have different carbon chain arrangements.
For (C), they share the same molecular formula i.e. ${\mathrm{C}}_4{\mathrm{H}}_{10}\mathrm{O}$, and have a similar arrangement of the carbon chain. 
Thus, (C) position isomer is the correct answer.