Concept:A subscript in a chemical formula shows the number of atoms of that element present in one molecule or formula unit.Explanation:The symbol O2 means that two oxygen atoms are joined together by a chemical bond.This pair of atoms forms a single particle called a molecule.So O2 represents a molecule made of two oxygen atoms, not two separate molecules or ions.It is the standard way to write oxygen gas because oxygen atoms are usually found in pairs.Thus, the symbol O2 indicates two atoms of oxygen joined as one molecule.Answer:B. atoms of oxygen.