The correct answer to this question is potassium phosphide. An IUPAC name is the set of rules that provide compounds with their name, set by the International Union of Pure and Applied Chemistry. Such names are published in the Nomenclature of Organic Chemistry. For the nameing conventions in chemsitry, the number of prefixes, suffixes, and infixes are used to describe the type and position of functional groups in the compound.
K3P is a binary ionic compound. Potassium almost always has a charge of + 1, whereas Phosphorus has charges of + 3, -3, + 5. For this example, Phosphorus will act as an Oxidizing agent with Potassium, thus it will take electrons from Potassium. This results in a negative three charge. Thus, for K3P, this becomes potassium phosphide.
