Oxidation-reactions are equations where some species release electrons and other species gain electron.
So two reactions happen simultaneously the oxidation of one species and the reduction off the other species.
In the oxidations, the species release electrons and, consequently, increase their oxidation number.
In the reductions, the species gain electrons and, consequently, decrease their oxidation number.
This last reaction, reduction, is what the above equation represents: Sn 4+ is gaining 2 electrons and so its oxidation number decrease from 4+ to 2+.
In, conclusion that is a reduction equation (option d)
