Which of the following pairs of elements could react to form an ionic compound? a. potassium and calcium b. sulfur and oxygen c. hydrogen and oxygen d. fluorine and copper

The correct option is FLUORINE AND COPPER.
An ionic compound is usually formed by the combination of a metal and a non metal, the metal usually act as an electron donor while the non metal act as an electron acceptor. Thus, in ionic compounds, there is total transfer of electrons from the metal to the non metal. In the question given here, copper is the metal while the fluorine is the non metal.