Explanation:
Since the oxidation state of Mg is changed to +2 from 0, it is oxidized. (as it is increased)
Similarly, oxidation state of Cu is changed to 0 from +2, it is reduced. (as it is decreased)
Since oxidation & reduction occur at the same time, it is redox reaction.
As Mg is being oxidized, other one must be the oxidizing agent.
Cu is the oxidizing agent
