Answer

The molecular masses of the given compounds are in the range 44 to 46 . ${CH_3} {CH_2} {OH}$ undergoes extensive intermolecular ${H}$-bonding, resulting in the association of molecules. Therefore, it has the highest boiling point. ${CH_3} {CHO}$ is more polar than ${CH_3} {OCH_3}$ and so ${CH_3} {CHO}$ has stronger intermolecular dipole - dipole attraction than ${CH_3} {OCH_3}. $ $ {CH_3} {CH_2} {CH_3}$ has only weak van der Waals force. Thus, the arrangement of the given compounds in the increasing order of their boiling points is given by:

${CH_3} {CH_2} {CH_3}<{CH_3} {OCH_3}<{CH_3} {CHO}<{CH_3} {CH_2} {OH}$