Increasing order of ionic radii of the followings:

(A) $Eu^{3+}$
(B) $Lu^{3+}$
(C) $Y^{3+}$
(D) $La^{3+}$

Choose the correct answer from the options given below:

(B) < (C) < (A) < (D)

The correct answer is Option (3) → (B) < (C) < (A) < (D)
The increasing order of ionic radii is - Lu³⁺ < Y³⁺ < Eu³⁺ < La³⁺.

1. Lanthanide contraction:
   In the lanthanide series, the ionic radius decreases from La³⁺ to Lu³⁺ because the 4f electrons do not shield the nuclear charge effectively, causing a stronger attraction between the nucleus and electrons.

2. Therefore:

   • Lu³⁺ (last lanthanide) has the smallest radius.

   • Eu³⁺ is larger than Lu³⁺ but smaller than La³⁺.

   • La³⁺ (first lanthanide) has the largest radius among the lanthanides listed.

3. Y³⁺ comparison:
   Y³⁺ has a radius slightly larger than Lu³⁺ but smaller than Eu³⁺, because its size is comparable to the heavier lanthanides due to lanthanide contraction.

Thus the increasing order becomes:
Lu³⁺ < Y³⁺ < Eu³⁺ < La³⁺
or
(B) < (C) < (A) < (D).