Lanthanoid contraction results in almost similar radii of Zr and Hf,…

The correct answer is Option (2) → 2^nd and 3^rd d series

The d-block elements form four transition series, according to the d subshell being filled:

• First (3d) series: Sc (Z = 21) to Zn (30)

• Second (4d) series: Y (39) to Cd (48)

• Third (5d) series: La (57) and Hf (72) to Hg (80)

• Fourth (6d) series: Ac (89) and Rf (104) onwards

So Zr and Hf belong to the 2^nd and 3^rd d series respectively.

NCERT (2026-27 edition):
Atomic and Ionic Sizes – …