In an aqueous mixture of aluminum, lead, and iron salts, which of these will be reduced first upon application of an electric current?

(a) al3+ + 3e− → al (s) eo = −1.66v
(b) pb2+ + 2e− → pb (s) eo = −0.13v
(c) fe2+ + 2e− → fe (s) eo = −0.45v
(d) fe3+ + 3e− → fe (s) eo = −0.036v

D) Fe3+ + 3e− → Fe (s) Eo = −0.036V

Explanation:

In electrolysis we have to add  electrical energy for redox reactions non-spontaneous because ΔºG is positive.

These  4 reduction reactions  are all non-spontaneous because their reduction potentials are negative   (ΔGº = - nFεº , ΔGº will be positive). So the ion most easily reduced is the least negative, in this case  Fe3+ + 3e− → Fe (s) Eo = −0.036V.