Consider two particles of different masses. In which of the following situations, the heavier of the two
particles will have smaller de-Broglie wavelength? [WB JEE 2015] 

The de-Broglie wavelength is 

$\lambda =\frac{h}{mv}\Rightarrow m=\frac{h}{\lambda v}\Rightarrow m\propto \frac{1}{\lambda },\text{ if }\frac{h}{v}=\text{ constant }$

Again, $\frac{h}{\nu }$ will be constant only when $\nu$ is constant.

Thus, in present case the de-Broglie wavelength of heavier particle will be smaller, when the velocities of two particles will be same.