A pendulum bob has a speed \(4 \mathrm{~m} / \mathrm{s}\) at its lowest position. The pendulum is 1 m long. When the length of the string makes an angle of \(60^{\circ}\) with the vertical, the speed of the bob at that position is (acceleration due to gravity, \(g=10 \mathrm{~m} / \mathrm{s}^2, \cos 60^{\circ}=0 \cdot 5\) )

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.