Which of the following lanthanoids shows the +2 oxidation state?

The correct answer is Option (1) → Eu

NCERT (2026-27 edition):
4.5.3 Oxidation States – “… However, occasionally +2 and +4 ions in solution or in solid compounds are also obtained. This irregularity (as in ionisation enthalpies) arises mainly from the extra stability of empty, half-filled or filled f subshell. … Eu²⁺ is formed by losing the two s electrons and its f⁷ configuration accounts for the formation of this ion.”