Given below are the pairs of group 13 element showing their relation in terms of atomic radius.
\((\mathrm{B} \lt \mathrm{Al}),(\mathrm{Al} \lt \mathrm{Ga}),(\mathrm{Ga} \lt \mathrm{In}) \text { and }(\mathrm{In} \lt \mathrm{Tl})\)
Identify the elements present in the incorrect pair and in that pair find out the element (X) that has higher ionic radius \(\left(\mathrm{M}^{3+}\right)\) that the other one. The atomic number of the element \((\mathrm{X})\) 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.