Answer
$Cu^{2+} (aq) + 2e^{-} -> Cu (s)$
This cathode reaction would produce a battery with the highest voltage.
Work Step by Step
In the anode, we have Zn, which is oxidized to $Zn^{2+}$. So, we need metal that is less active than Zn in the metal activity series (below Zn in the metal activity series) for a spontaneous reaction. Also, to produce highest voltage we have to select the metal which is least active. Now, if we look at the metal activity series, Cu is least active than the other three metals in the options i.e. Mg, Pb and Cr. So, $Cu^{2+} (aq) + 2e^{-} -> Cu (s)$ This reaction in the cathode would produce a battery with the highest voltage.
