The correct order of ionic radii of Y^3+, La^3+, Eu^3+ and Lu^3+ is (Atomic nos. Y = 39, La = 57, Eu = 63, Lu = 71)

The correct answer is (d) La3+ < Eu3+ < Lu3+ < Y3+. This is because, in general, as the size of atomic number increases across a row of the periodic table (in this case, the lanthanide series), the size of the ionic radius typically decreases. Therefore, the ionic radius of Y3+ (atomic number 39) is larger than the ionic radius of La3+ (atomic number 57), which is larger than the ionic radii of Eu3+ (atomic number 63) and Lu3+ (atomic number 71). Therefore, La3+ < Eu3+ < Lu3+ < Y3+ is the correct order of ionic radii.