How can I determine the overall electrochemical reaction that proceeds spontaneously?

Question

Cd2+ | Cd, Co2+ | Co
1. Cd+Co2+ −�¨Cd2++Co
2. Co+Cd2+ −�¨Co2++Cd
(1. and 2. are answer choices)

Answer 1

Look up reduction potentials for
Cd^2+ + 2e==> Cd and
Co2^+ + 2e ==> Co.

Look at which one of the two will give a positive voltage. That is the one that will proceed spontaneously.