JEE Main · 2019 · 12 Jan 2019 · Shift 2 · d and f Block Elements

These are lanthanides, whose radii shrink across the series as the poorly shielding $4f$ electrons let the nuclear pull tighten the atom $-$ the lanthanide contraction. Europium stands out as anomalously large because its stable half-filled $4f^7$ configuration lowers the effective nuclear charge felt by the bonding electrons. The resulting order is $\mathrm{Eu}>\mathrm{Ce}>\mathrm{Nd}>\mathrm{Ho}$.