(d) $T = 2\pi\sqrt{\frac{l}{g}};$ $T = 2\pi\sqrt{\frac{1}{g + g/3}} = 2\pi\sqrt{\frac{3l}{4g}} = \left( \sqrt{\frac{3}{4}} \right)T^{'}$ or $t^{'} = \frac{2T}{\sqrt{3}}$
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.