Number of electrons transferred by each formula unit of ferrous oxalate acting as reductant in acidic solutions

${Fe}^{2+}+C_2{O}_4{}^{2-}⟶{Fe}^{3+}+2{CO}_2+3e^{-}$

Iron (II) undergo oxidation to form iron (III) which involves 1 electron  Oxalate (III) undergo oxidation to form carbon dioxide where carbon oxidation state is +IV which involves 1 electron. There are two moles of carbon dioxide formed and hence 2 electrons involved.

Overall the reaction involves 3 electrons  

Hence the correct option is (C).