Answer:
Ba
Explanation:
Let us consider the electrode potential of one of the options listed in the question,
Ba2+(aq) + 2 e- -----> Ba(s). -2.90V
It is very important to recall that a metal can only displace another metal from aqueous solution if that metal has a more negative reduction potential that the metal it displaced. This implies that if there are two metals A and B, A can only displace B from an aqueous solution of B if A has a more negative reduction potential than B.
The reduction potential of Mg^2+ is -2.37V. This is less negative than that of Ba^2+ which is -2.90V. Hence barium can reduce Mg^2+ to metallic Mg.
