The ion which is most easily reduced is the cobalt (II) ion (Co²⁺).
What is reduced species?
Reduced species are those species which itself gets reduced by the addition of electrons in it, and known as the oxidizing agents as it helps in the oxidation reaction.
Among the given species according to the electrochemical series strength of the metals as oxidizing agent increases in the following manner:
Ca²⁺ < Mg²⁺ < Zn²⁺ < Co²⁺
So, Co²⁺ is most easily reduced as compared to others.
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