The equation that represents a spontaneous reaction is Ni + Mn2+ → Ni2+ + Mn
A spontaneous reaction serves as the reaction that is capable of proceeding in a given direction without outside source of energy.
It is a reaction that is capable of occurring on it's own, without external force. Example of this reaction is the combustion reaction.
Therefore, option 4 is a spontaneous reaction since it's a reaction between Nikel and magnesium and occur on its own.
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