We must keep in mind that matter is neither created nor destroyed, it is only transformed or rather in this case reacts. So the atoms that are in the products must be conserved in the reactants although ordered in a different way.
We have on the product side 3 hydrogen atoms, 1 phosphorus atom, and 4 oxygen atoms.
In the reactants we have 1 phosphorus atoms and 4 oxygen atoms, the missing atom would be hydrogen and it must be in equal quantity to the hydrogen atoms of the products, that is, 3 hydrogen atoms.
Therefore the reactant missing from the equation is three hydrogen atoms. The answer is the third option
