A $U$-shaped tube is partially filled with an incompressible liquid of density $1.2 \mathrm{g} {\mathrm{cm}}^{-3}.$ Oil which does not mix with the liquid is next poured into the left side of the $U$-tube until the liquid rises by, $15 \mathrm{cm}$ on the right side of $U$-tube. If the density of the oil is $0.9 \mathrm{g} {\mathrm{cm}}^{-3},$ the oil level will stand higher than the liquid level of the right side of $U$-tube by

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.