a
\(T^{\prime}=2 \pi \sqrt{\frac{\ell^{\prime}}{g}} \text { where } \ell^{\prime}=\frac{\ell}{2}\)

\(T=2 \pi \sqrt{\frac{\ell}{g}}\)

\(T^{\prime}=\frac{x}{2} T\)

\(2 \pi \sqrt{\frac{\ell}{2 g}}=\frac{x}{2} 2 \pi \sqrt{\frac{\ell}{g}}\)

\(\frac{1}{\sqrt{2}}=\frac{x}{2} \Rightarrow x=\sqrt{2}\)