An electron of mass \( 9.1 \times 10^{-31} \text{ kg} \) carrying charge \( 1.6 \times 10^{-19} \text{ C} \), is moving horizontally with a speed of \( 4 \times 10^6 \text{ m/s} \) in a uniform magnetic field of magnitude \( 8 \times 10^{-4} \text{ T} \) acting vertically upward. The radius of its circular path is,

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.