The above reaction shows that P₄ undergoes reduction and oxidation simultaneously showing an example of disproportionation reaction. The oxidation number in P₄ is zero as there are all elemental toms of phosphorous bonded in a pyramidal geometry. The oxidation reaction in results in the formation of Hypophosphite ion H₂PO^-₂, which has an oxidation state of

Thus showing oxidation of P atom from 0 to +1. In case of phosphine the hydrogen involves has +1 ON number thus P having ON of -3, showing P is undergoing reduction.

In option (iv) it is stated that hydrogen is neither oxidant nor reductant but for H₂O, H₂ being in +1 state throughout the entire reaction it neither undergoes oxidation nor reduction, thus Option (iv) is correct.