(b) $T = 2\pi\sqrt{\frac{m}{K}}.$ Also spring constant $(K) \propto \frac{1}{Length\ (l)},$
when the spring is half in length, then $K$ becomes twice
$\therefore T^{'} = 2\pi\sqrt{\frac{m}{2K}} \Rightarrow \frac{T^{'}}{T} = \frac{1}{\sqrt{2}} \Rightarrow T^{'} = \frac{T}{\sqrt{2}}$