In hydrogen atom in its ground state, the first Bohr orbit has radius ' \(r_1{ }^{\prime}\). When the atom is raised to one of its excited states, the electrons orbital velocity becomes one-third. The radius of that orbit 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.