(c)
$U_{av} \propto \frac{1}{\sqrt{M}}\$at constant temperature
$\frac{U_{av}\left( {SO}_{2} \right)}{U_{av}\ \left( {CH}_{4} \right)} = \sqrt{\frac{M_{{CH}_{4}}}{M_{{SO}_{2}}}} = \sqrt{\frac{16}{64}} = \frac{1}{2}$
$U_{{SO}_{2}}\ :U_{{CH}_{4}} = 1\ :2$