Which pair does not represent isomers?

Isomers are each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and other properties. 

So, out of the given option, ${\mathrm{CH}}_3\mathrm{CHO}, {\mathrm{CH}}_3{\mathrm{COCH}}_3$ are not isomers as the molecular formula is different in both the molecules. 

So, option C) is correct.