# One among the following pairs of compounds is not isomers:

$(a)\;(CH_3)_2CH - O -C_2H_5\;and \;CH_3(CH_2)_2 -O -C_2H_5 \\(b)\;CH_3CH_2CO_2H \;and\; CH_3CO_2CH_3 \\ (c)\;CH_3COCH_3 \;and\; CH_3CH_2CH_2OH \\ (d)\;CH_3CH_2NO_2 \;and\; NH_2CH_2COOH$

The phenomenon in which different organic compounds having same molecular formula possess different physical and chemical properties is called isomerism.
In (c) , the compounds have different molecular formula.
So, $CH_3COCH_3 \;and\; CH_3CH_2CH_2OH$ is correct
Hence c is the correct answer.