Which two substances react spontaneously? Which two substances react spontaneously?
(A) $\ce{Ag}$ and $\ce{Cu}$
(B) $\ce{Ag+}$ and $\ce{Cu}$
(C) $\ce{Ag}$ and $\ce{Cu^2+}$
(D) $\ce{Ag+}$ and $\ce{Cu^2+}$
$\tiny{\text{Taken from 2012 Chem Olympiad}}$

The given answer is B, yet I am unclear as to why. I believe it has something to do with reduction potentials/redox or the gibbs free energy. Can someone please explain?
From the faint outline of electrochemical series I remember $Cu$ is above $Ag$. The substance which is above is able to displace the substance below from its aqueous solution(obviously lower substance is in ionic form).

Hence $(B)$ is true.

We were made to learn this outline in lower class :

K Na Ca Mg Al Zn Fe Pb H Cu Hg Ag Au Pt