Which of the following is a molecule of an element?

An element of a pure substance contains only one kind of atom. Molecule is a group of two or more atoms that are chemically bonded together, i.e. tightly held together by attractive forces. Atoms of the same element or different elements can join to form molecules. The same type of atoms constitutes the molecules of an element. A molecule of ozone (O₃) consists of three atoms of oxygen and thus, it is a molecule of an element. Sugar (C₁₂H₂₂O₁₁), sodium chloride (NaCl) and salt (NaCl) consist of atoms of different elements joined together in definite proportions. Thus, they are molecules of compounds.