Two cells having unknown e.m.f.s \(\mathrm{E}_{1}\) and \(\mathrm{E}_{2}\left(\mathrm{E}_{1}>\mathrm{E}_{2}\right)\) are connected in potentiometer circuit so as to assist each other. The null point obtained is at 490
\(\mathrm{cm}\) from the higher potential end. When cell \(\mathrm{E}_{2}\) is connected so as to oppose cell \(\mathrm{E}_{1}\),
Ithe null point is obtained at \(90 \mathrm{~cm}\) from the same end. The ratio of the e.m.f.s of
two cells \(\left(\frac{\mathrm{E}_{1}}{\mathrm{E}_{2}}\right)\) 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.