c
For a simple pendulum \(\mathrm{T}=2 \pi \sqrt{\frac{\mathrm{L}}{\mathrm{g}_{\mathrm{err}}}}\)

Situation \(1\): when pendulum is in air \(\rightarrow g_{\text {eff }}=g\)

Situation \(2\) :when pendulum is in liquid 

\(\rightarrow g_{e f}=g\left(1-\frac{\rho_{\text {liquid }}}{\rho_{\text {body }}}\right)=g\left(1-\frac{1}{16}\right)=\frac{15 g}{16}\)

So, \(\frac{\mathrm{T}^{\prime}}{\mathrm{T}}=\frac{2 \pi \sqrt{\frac{\mathrm{L}}{15 \mathrm{g} / 16}}}{2 \pi \sqrt{\frac{\mathrm{L}}{\mathrm{g}}}}\)

\(\Rightarrow \mathrm{T}^{\prime}=\frac{4 \mathrm{T}}{\sqrt{15}}\)