Explanation

(c) $\lambda = \frac{h}{\sqrt{2mE}} = \frac{h}{\sqrt{2m}}.\frac{1}{\sqrt{E}}.$ Taking log of both sides
$\log{\lambda = \log{\frac{h}{\sqrt{2}m} + \log{\frac{1}{\sqrt{E}} \Rightarrow \log{\lambda = \log{\frac{h}{\sqrt{2}m} - \frac{1}{2}\log E}}}}}$
$\Rightarrow \log{\lambda = - \frac{1}{2}\log{E + \log\frac{h}{\sqrt{2m}}}}$
This is the euation of straight line having slope $( - 1/2)$ and positive intercept on log $\lambda$ axis