JEE Main · 2004 · 24 Apr 2004 · Offline · d and f Block Elements

Cerium's usual oxidation states are $+3$ and $+4$, and cerium(IV) is a well-known, strong oxidising agent, the $\mathrm{Ce}{}^{4+}/\mathrm{Ce}{}^{3+}$ couple sitting around $+1.7\text{V}$. Crucially, $\mathrm{Ce}{}^{4+}$ is perfectly familiar in aqueous solution, so the claim that the $+4$ state is unknown in solution is the false one. That the $+3$ state is the more stable of the two is itself a correct remark.