Answer: Ni
Explanation: Oxidation-reduction reaction or redox reaction is defined as the reaction in which oxidation and reduction reactions occur simultaneously.
Oxidation reaction is defined as the reaction in which a substance looses its electrons. The oxidation state of the substance increases.
\(Ni\rightarrow Ni^{2+}+2e^-\)
Reduction reaction is defined as the reaction in which a substance gains electrons. The oxidation state of the substance gets reduced.
\(Sn^{4+}+2e^-\rightarrow Sn^{2+}\)
The oxidation state of nickel increases from 0 to +2, it is getting oxidized and it undergoes oxidation reaction.
