Answer : Hg (Mercury)
Explanation : In the given series of elements Na falls after the Cu in the reactivity series, for Pb it also falls after Cu, and for Mg it is the same.
Only Hg which is mercury can spontaneously donate its electrons to copper in the solution because it falls before Cu in the reactivity/activity series.
Explanation:
According to the reactivity series, lead, magnesium, and sodium are all reactive than copper.
Hence, when copper is added to a solution along with anyone of these metals (lead, magnesium, and sodium) then copper will be displaced by them.
Whereas mercury (Hg) being less reactive than copper will not be able to spontaneously donate electrons to copper in solution.
