A ball of mass \(1 \mathrm{~g}\) having a charge of \(20 \mu \mathrm{C}\) is tied to one end of a string of length \(0.9 \mathrm{~m}\) can rotate in a vertical plane in a uniform electric field \(100 \mathrm{NC}^{-1}\) directed upwards. The minimum horizontal velocity that must be given to the ball at the lowest position so
that it completes the vertical circle is (Let, \(\mathrm{g}=10 \mathrm{~ms}^{-2}\))

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.