Parallel Plate Capacitor question from JEE Main 2019 (10 Apr, Shift 2)

Answer: (c)

1. On the bob: weight $mg$ downward and $qE$ horizontal, perpendicular to each other.
2. Effective gravity: $g_{eff} = \sqrt{g^2 + \left(\frac{qE}{m}\right)^2}$
3. $T = 2\pi\sqrt{\frac{L}{g_{eff}}}$
4. So the answer is (c) $2\pi\sqrt{\frac{L}{\sqrt{g^2 + \frac{q^2E^2}{m^2}}}}$.