How many electrons are involved in the following redox reaction ? $\;Cr_{2}O_{7}^{2-}+Fe^{2+}+Cr_{2}O_{4}^{2-} \to Cr^{3+} + Fe^{3+} + CO_{2}\;$ (unbalanced)

Question

$(a)\;3\qquad(b)\;4\qquad(c)\;6\qquad(d)\;5$