A siren placed at a railway platform is emitting a sound of frequency \(5 \mathrm{kHz}\). A passenger sitting in a moving train \(A\) records the frequency of the siren as \(5.5 \mathrm{kHz}\).
During his return journey by \(\operatorname{train} B\) he records the frequency of the siren as \(6 \mathrm{kHz}\). The ratio of the speed of \(\operatorname{train} B\) to that of \(\operatorname{train} A\) 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.