the dipole/induced-dipole and dipole-dipole attractive forces give alcohols (of similar sizes) a higher bp than alkanes like c2h6. therefore c2h6 should have the lowest bp.
next you would look for the ones with hydrogen bonding, since it takes a large amount of energy to break these. therefor ch3cooh and c2h5oh will have a higher bp than ch3cho. (in ch3cho, the last hydrogen is attached to the second carbon.. not the oxygen atom)
because more energy is needed to break the double bond in ch3cooh, it will have the highest bp