All four compounds have molar mass $44$-$46\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$. The correct order of boiling points is

With similar molar masses, the strongest attraction decides. Ethanol has hydrogen bonds ($78^{\circ }\mathrm{C}$). Ethanal is strongly polar ($\mu \approx 2.7\mathrm{D}$, b.p. $20^{\circ }\mathrm{C}$), dimethyl ether is weakly polar ($1.3\mathrm{D}$, b.p. $-24^{\circ }\mathrm{C}$), and propane is non-polar ($-42^{\circ }\mathrm{C}$). The order is ethanol > ethanal > ether > propane.