In a voltaic cell there are two electrodes: anode and cathode.
On the cathode, the reaction that occurs is reduction, which means that the metal gains electrons.
On the anode the reaction is oxidation, which means that the metal loses electrons.
In this voltaic cell the reaction on the anode is:
Zn --> Zn 2+ + 2e-, i.e. option C.
Answer: option C.