What is the correct order of decreasing bond strength?

Ionic bonds result via the transfer of electrons from a highly electropositive element (e.g., Na) to an electronegative element (e.g., Cl). Thus, NaCl consists of Na+ and Cl– ions, each of which is stabilized by a complete octet of electrons in its outer shell.

Covalent bonds result from sharing of electrons. Although covalent bonds between nonmetals are the strongest chemical bonds, they are significantly weaker than ionic bonds.

Intermolecular hydrogen bonds can be formed between one molecule containing relatively acidic protons and another molecule that possess one or more electronegative elements (N, O and/or F). Hydrogen bonds are considerably weaker than covalent bonds but nevertheless are important as evidenced by the high boiling point of water.

London dispersion forces result from weak temporary dipole-dipole interactions between otherwise nonpolar molecules. The low boiling points associated with low molecular weight hydrocarbons reflect the operation of weak intermolecular dispersion forces.