Final answer:
In the given chemical reaction, none of the elements increase their oxidation number; phosphorus (P), potassium (K), oxygen (O), and hydrogen (H) all maintain their respective oxidation states.
Step-by-step explanation:
The element that increases its oxidation number in the reaction 3KOH + H₃PO₄ → K₃PO₄ + 3H₂O is phosphorus (P). In the reactant H₃PO₄, phosphorus has an oxidation state of +5 (as it is bonded to four oxygens and one hydrogen). In the product K₃PO₄, phosphorus also has an oxidation state of +5, so there's no change to its oxidation state; thus, we do not consider it to be oxidized.
However, if we look closely, none of the other elements change their oxidation states either. Potassium, being an alkali metal, maintains a +1 oxidation state in both KOH and K₃PO₄. Oxygen is typically -2 in compounds except in peroxides, and this does not change here. Hydrogen is +1 when bonded to nonmetals, including in water (H₂O) and in H₃PO₄. Therefore, none of the element's oxidation numbers actually increase in this reaction.