Which of the following is a binary ionic compound?

A binary ionic compound is defined as a compound composed of two different elements, where one element is a metal and the other is a nonmetal, forming ionic bonds. In this case, K2O fits the criteria perfectly. It consists of potassium (K), a metal, and oxygen (O), a nonmetal. Potassium donates electrons to oxygen, resulting in the formation of K^+ and O^2− ions, which combine in a 2:1 ratio to form potassium oxide.

Silicon dioxide (SiO2) and phosphorus trioxide (P2O3) are examples of molecular compounds composed entirely of nonmetals, while copper(III) sulfite (CuSO3) contains a polyatomic ion (sulfite) and is not classified as a binary compound due to the presence of more than two elements. Thus, K2O is correctly identified as a binary ionic compound, distinguishing it from the other options based on its composition and the nature of the bonds formed.