The correct answer is Na+, reduction is the gaining of electrons by ions, the ease at which an ion or an atom gains electrons depends on its position in the electrochemical series. Electrochemical series arranges atoms of elements in order, based on the ease of gaining electrons. In this case, Na+ can easily gain electrons compared to the other ions given. We can also look at it based on the electromotive force of each (EMF); +0.77 Vb. -0.25 Vc. -2.71 Vd. +0.15; this evidently shows that Na+ can undergo reduction easily compared to the other ions