Which of the following has the highest standard electrode potential?

The highest standard electrode potential corresponds to the tendency of a species to be reduced, meaning it is more likely to gain electrons. Among the options provided, mercury (Hg) has a higher standard electrode potential compared to silver (Ag), potassium (K), and magnesium (Mg).

Mercury typically has a standard electrode potential of +0.85 V when considered as Hg²⁺/Hg, while silver's standard potential is +0.80 V for Ag⁺/Ag. Potassium and magnesium have much lower electrode potentials due to their higher reactivity and tendency to lose electrons rather than gain them, with values significantly below zero.

Thus, because of mercury's favorable position in the electrochemical series, it retains the highest standard electrode potential among the listed elements. This trait makes mercury relatively more stable in solution compared to the others, which are more reactive and have lower potentials.