Given
${\mathrm{E}}_{{\mathrm{Cl}}_2/{\mathrm{Cl}}^{-}}^{\mathrm{o}}=1.36\mathrm{V},\mathrm{ }{\mathrm{E}}_{{\mathrm{Cr}}^{3+}/\mathrm{Cr}}^{\mathrm{o}}=\mathrm{ }-0.74\mathrm{V}$
${\mathrm{E}}_{{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}/{\mathrm{Cr}}^{3+}}^{\mathrm{o}}=1.33\mathrm{V},\mathrm{ }{\mathrm{E}}_{{\mathrm{MnO}}_4^{-}/{\mathrm{Mn}}^{2+}}^{\mathrm{o}}=1.51\mathrm{V}.$
Among the following, the strongest reducing agent is:

Strongest reducing agent has the highest tendency to get oxidised. Higher the oxidation potential, higher is the tendency to get oxidised. Reduction potential of ${\mathrm{Cr}}^{3+}$ is least (${\mathrm{E}}_{{\mathrm{Cr}}^{3+}/\mathrm{Cr}}^{\mathrm{o}}=\mathrm{ }-0.74\mathrm{V}$) which indicates that oxidation potential of $\mathrm{Cr}$ is highest (${\mathrm{E}}_{\mathrm{Cr}/{\mathrm{Cr}}^{3+}}^{\mathrm{o}}=\mathrm{ }0.74\mathrm{V}$). Thus, $\mathrm{Cr}$ is strongest reducing agent.