One among the following pairs of compounds are not isomers ?

Isomers are molecules or polyatomic ions having the same number of atoms in each element as their molecular formulas, but different configurations of atoms in space. Isomerism is the possibility or presence of isomers. Isomers don't always have the same chemical or physical characteristics.

Hence, correct option is: (C) ${\mathrm{CH}}_3{\mathrm{COCH}}_3\text{ and }{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{OH}$