Answer
$Pb^{2+}$ is the metal cation that has the greatest tendency to be reduced.
Work Step by Step
From the list of the metal cations $(Pb^{2+}, Cr^{3+}, Fe^{2+}, and Sn^{2+})$ the metal cation which has the greatest tendency to be reduced or gain electrons is $Pb^{2+}$ since $Pb^{2+}$ wants to gain electrons (i.e. be reduced) and this is because it is lower on the activity series than the other options listed.