Which of the following is an example of a covalent bond?

A covalent bond is formed when two atoms share one or more pairs of electrons. This typically occurs between nonmetals, which have similar electronegativities and are more inclined to share electrons than to transfer them completely.

In the case of water, which consists of two hydrogen atoms and one oxygen atom, the hydrogen atoms share their electrons with the oxygen atom. This sharing allows each hydrogen to attain a stable electron configuration, as oxygen will also complete its valence shell through this sharing mechanism. The bonds that form between the hydrogen and oxygen atoms in water are thus classified as covalent bonds, specifically they are polar covalent due to the difference in electronegativity between the oxygen and hydrogen atoms.

The other options represent ionic bonds where electronegative differences are significant enough to cause the transfer of electrons rather than sharing. In sodium chloride, calcium oxide, and magnesium sulfide, the bonding involves the transfer of electrons from one atom to another, resulting in the formation of oppositely charged ions.