$\begin{array}{1 1} \text{(a) Methane and argon have weak dispersion forces so their boiling point is low} \\ \text{(b) Diethyl ether has a dipole moment, and the dipole-dipole forces accounts for its moderately high boiling point} \\ \text{(c) Water and ethanol have strong hydrogen bonding and hence they possess high boiling point} \\ \text{(d) Mercury has weak metallic bonding which is responsible for its low boiling point}
\end{array} $
