Red phosphorus is a chain compound of the P4 molecule. In this compound each phosphorus is bonded to three phosphorus atoms. The possible oxidation states of phosphorus are +/- 3,5,4.
But, we must keep in mind that the oxidation number of the atoms of a free element is zero. Thus, each atom in the P4 chain has an oxidation number of zero. This is because phosphorus is not combined.
Answer: 4. 0
