Preventing oxidation of copper(I)
We need a better reducing agent (that itself oxidises) than $\ce {Cu^+}$.
So choose the ones which have greater reduction potential than $\ce {Cu^+}$.
It isn't $\ce {Mg^2+}$ as it cannot oxidise any further hence it has a very low(negative) reduction potential, so it cannot stop $\ce {Cu^+}$ from oxidising.