A hydrogen molecule and helium atom are moving with the same velocity. Then the ratio of their de Brogile wavelength is

$\mathrm{\lambda } =\frac{\mathrm{h}}{\mathrm{mv}}$

 velocity is same for Hydrogen and helium atoms

$$\begin{gathered} \mathrm{\lambda } =\frac{\mathrm{h}}{\mathrm{m}} \\ \mathrm{\lambda \alpha }\frac{1}{\mathrm{m}} \end{gathered}$$

$\frac{ {\mathrm{\lambda }}_{\mathrm{H}}}{ {\mathrm{\lambda }}_{\mathrm{He}}} =\frac{{\mathrm{m}}_{\mathrm{He}}}{{\mathrm{m}}_{\mathrm{H}}}$

Mass of H= 2 units 

Mass of He =4 units

$\frac{ {\mathrm{\lambda }}_{\mathrm{H}}}{ {\mathrm{\lambda }}_{\mathrm{He}}} =\frac{4}{2}=\frac{2}{1}$