JEE Main 2025 Jan 24 Shift 1, Chemistry Q6: f – Block Elements

A strong oxidising agent is itself readily reduced. Among lanthanides, the stable oxidation state is +3; ions in non-+3 states that can revert to +3 by gaining (or losing) electrons act as strong oxidisers (or reducers).

$\mathrm{L}{\mathrm{u}}^{3+}$ and $\mathrm{C}{\mathrm{e}}^{3+}$ are already in the stable +3 state, so they have no driving force to be reduced.

$\mathrm{E}{\mathrm{u}}^{2+}$ is in a lower-than-stable state; it tends to lose an electron (reducing agent), not gain one.

$\mathrm{T}{\mathrm{b}}^{4+}$ is in a higher-than-stable state and is strongly driven to pick up an electron and return to the stable $\mathrm{T}{\mathrm{b}}^{3+}$ $\Rightarrow$ powerful oxidising agent.

$\therefore$ Option (3).