b
The regular decrease in the radil of lanthanide ions from \(L a^{3+}\) to \(L u^{3+}\) is known as lanthanides contraction. It is due to the greater effect of the increased nuclear charge than that of screening effect (shielding effect). As a result of lanthanide contraction, the atomic radii of element of \(4 \mathrm{d}\) and \(5 \mathrm{d}\) come closetr, so the properties of \(4 \mathrm{d}\) and \(5 \mathrm{d}\) -transition element shows the similarities