For the four successive transition elements (Cr, Mn, Fe and Co),...

This can be understood on the basis of $E^{{}^{\circ}}$ values for $M^{2+} / M$. $$\begin{array}{lrrcr}E \% V & \mathrm{Cr} & \mathrm{Mn} & \mathrm{Fe} & \text{ Co } \\ M^{2+} / M & -0.90 & -1.18 & -0.44 & -0.28\end{array}$$E^{{}^{\circ}}value for $\mathrm{Mn} is more negative than expected from general trend due to extra stability of half-filled \mathrm{Mn}^{2+}$ion. Thus the correct order should be, $\mathrm{Mn}>\mathrm{Cr}$>$\mathrm{Fe}>\mathrm{CO}an$ examination of $E^{{}^{\circ}}$ values for redox couple $M^{3+} / M^{2+}$ shows that $\mathrm{Cr}^{2+} is strong reducing$\operatorname{agent}\left(E_{M^{3+}}^{{}^{\circ}} / M^{2+}=0.41 $\mathrm{~V}$\right)$and liberates\mathrm{H}_{2}$ from dilute acids. 2 $\mathrm{Cr}^{2+}($a q)+2 \mathrm{H}^{+}(a q) → 2 $\mathrm{Cr}^{3+}($a $q)+\mathrm{H}_2 \uparrow($g) $\therefore$ The correct order is $\mathrm{Mn} \g$t $\mathrm{Fe} \g$t $\mathrm{Cr} \g$t $\mathrm{Co}.$