Which of the following acts as a strong reducing agent? (Atomic...

A reducing agent is one of the reactants of an oxidation-reduction reaction which reduces the other reactant by giving out electrons to the reactant. If the reducing agent does not pass electrons to other substances in a reaction, then the reduction process cannot occur.

Europium (atomic number $63$) has the electronic configuration \left[Xe\right]4{f}^{7}6{s}^{2}$. It loses two electrons from the 6s energy level and attains a highly stable, half-filled$4{f}^{7}$configuration, and hence it readily forms${Eu}^{2 +}$ion.${Eu}^{2 +}$then changes to the common oxidation states of lanthanides \left(+ 3\right)$ and forms ${Eu}^{3 +}$, acting as a strong reducing agent.

$\begin{cases}{Eu}^{+ 2} \to & {Eu}^{+ 3} + 1{e}^{-} \\ [ Xe ] 4{f}^{7}6{s}^{0} & [ Xe ] 4{f}^{6}6{s}^{0}\end{cases}$