Each of the given statements needs to be examined carefully to identify the correct one. Let's analyze them one by one: Option A: The green manganate ion shows diamagnetic nature but the permanganate ion exhibits paramagnetic nature. This statement is not correct because both the manganate ion (MnO42−) and the permanganate ion ( MnO4−) are paramagnetic in nature, as both contain unpaired electrons in their d-orbitals. Option B: Interstitial compounds of transition metals have lower melting points than that of pure transition metals and their compounds are chemically reactive. This statement is also not correct. Interstitial compounds typically have higher melting points compared to their pure metals due to the presence of small atoms (like hydrogen, carbon, or nitrogen) in the interstices of the metal lattice structure, which enhances the metallic bonding. Additionally, they are generally less reactive than their constituent metals. Option C: Cerium is a lanthanoid metal which exists in a stable oxidation state of +4 , besides exhibiting an oxidation state of +3 . This statement is correct. Cerium (Ce) indeed has two common oxidation states: +3 and +4 . The +3 oxidation state is more common for lanthanoids, but cerium is unique in that it can exist stably in the +4 oxidation state as well. Option D: Cr(VI) is more stable than W(VI) and hence acts as a good oxidising agent. This statement is not correct. In fact, W(VI) is generally more stable than Cr(VI). Chromium (VI) compounds are strong oxidizing agents, indicating they are less stable and readily reduced. Therefore, the correct statement is: Option C: Cerium is a lanthanoid metal which exists in a stable oxidation state of +4 , besides exhibiting an oxidation state of +3 .