Final answer:
NaCl is the molecule among the given options that is not formed by covalent bonding, but rather, it is formed by ionic bonding.
Step-by-step explanation:
Among the given options, the molecule that is NOT formed by covalent bonding is NaCl. The bond in Sodium Chloride (NaCl) is an ionic bond, not a covalent bond. Ionic bonds occur when a metal (Na in this case) transfers one or more electrons to a nonmetal (Cl in this case) to form ions, which are then attracted to each other. On the other hand, in covalent bonds, which we can observe in the molecules H2, H2O, CH4, and O2, atoms share electrons rather than transferring them.
Learn more about Ionic and Covalent Bonds