All the compounds have nearly the same atomic masses. Out of all these compounds propane is non polar and will have, as such, lowest boiling point. Dimethyl ether is polar and as such will have a higher boiling point than propane. Ethanol, more polar than dimethyl ether will have a higher boiling point than dimethyl ether. Ethanol will have the highest boiling point due to Hydrogen bonding. There for increasing order of boiling point is
CH3 - CH2 - CH3 < CH3OCH3 < CH3CHO < CH3CH2OH.
