The standard electrode potential (E^ - ) values of (Al^ 3+ / Al,...

More negative the value of standard reduction potential, higher is the reduction power. i.e. Reducing power $\propto \frac{1}{\text{ standard reduction potential }}$ Thus, the correct decreasing order of reducing power of the metal is \begin{aligned} & \begin{array}{llll} \mathrm{K} & > & \mathrm{Al} & > \end{array} \\ & \left(\mathrm{E}_{\mathrm{K}^{+} / \mathrm{K}}^0=-2.93 \mathrm{~V}\right) \quad\left(\mathrm{E}_{\mathrm{Al}^{3+} / \mathrm{Al}}^0=-1.66 \mathrm{~V}\right) \\ & \mathrm{Cr}>\mathrm{Ag} \\ & \left(\mathrm{E}_{\mathrm{Cr}^{3+} / \mathrm{Cr}}^0=-0.74 \mathrm{~V}\right) \quad\left(\mathrm{E}_{\mathrm{Ag}^{+}}^0 / \mathrm{Ag}=0.80 \mathrm{~V}\right) \\ & \end{aligned}$$$