Among the following lanthanoid ions, which one acts as a strong reducing agent?

Step-by-step solution

The common oxidation state of all lanthanoids is +3 . Eu ²⁺ has an electronic configuration of [ Xe ] 4f⁷ . Despite having a stable half-filled f -subshell, it readily loses one electron to achieve the most common and stable +3 oxidation state (forming Eu ³⁺ ). Therefore, it acts as a strong reducing agent. Ce ⁴⁺ and Tb ⁴⁺ tend to gain an electron to reach the +3 state, making them oxidising agents. Lu ³⁺ is already in the stable +3 state with a fully filled 4f¹⁴ configuration and does not act as a reducing agent.