Which of the following acts as a strong reducing agent? (Atomic number: \( \mathrm{Ce} = 58 \), \( \mathrm{Eu} = 63 \), \( \mathrm{Gd} = 64 \), \( \mathrm{Lu} = 71 \))
\( \mathrm{Lu^{3+}} \)
\( \mathrm{Ce^{4+}} \)
\( \mathrm{Gd^{3+}} \)
\( \mathrm{Eu^{2+}} \)