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