(d) For the given figure $f = \frac{1}{2\pi}\sqrt{\frac{k_{eq}}{m}} = \frac{1}{2\pi}\sqrt{\frac{2k}{m}}$ …(i)
If one spring is removed, then $k_{eq} = k$ and
$f^{'} = \frac{1}{2\pi}\sqrt{\frac{k}{m}}\ \ \ \ \ \ \ \ \ldots(ii)$
From equation (i) and (ii),
$\frac{f}{f^{'}} = \sqrt{2} \Rightarrow f^{'} = \frac{f}{\sqrt{2}}$