c
The \(+3\) oxidation state of lanthanide is most stable and therefore lanthanide in \(+4\) oxidation state has strong tendence to gain \(e ^{-}\) and converted into \(+3\) and therefore act as strong oxidizing agent.

\(eg Ce ^{+4}\) And there fore \(CeO _{2}\) is used to oxidized alcohol aldehyde and ketones.

Lanthanide in \(+2\) oxidation state has strong tendency to loss \(e ^{-}\) and converted into \(+3\) oxidation state therefore act as strong reducing agent.

\(\therefore EuSO _{4}\) act as strong reducing agent.