c
(c)

When bob is inserted, in liquid effective of is reduced because of the force of upthrust.

\(\rho_s\) is density of solid.

\(\rho_L\) is density of liquid.

\(V\) is volume of solid

and \(T=2 \pi \sqrt{\frac{1}{g}}\)

\(T_{\text {new }}=2 \pi \sqrt{\frac{1}{g_{\text {new }}}}\)

\(T_{\text {new }}=2 \pi \sqrt{\frac{1}{g-\frac{\rho_L}{\rho_S} g}}\)

\(T_{\text {new }}=2 \pi \sqrt{\frac{1 \times 12}{g \times 11}}\)

or \(T_{\text {new }}=\sqrt{\frac{12}{11}} T\)