(b) $\lambda_{photon} = \frac{hc}{E}\ and\ \lambda_{electron} = \frac{h}{\sqrt{2mE}}$
$\Rightarrow \frac{\lambda_{photon}}{\lambda_{electron}} = c\sqrt{\frac{2m}{E}} \Rightarrow \frac{\lambda_{photon}}{\lambda_{electron}} \propto \frac{1}{\sqrt{E}}$