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

A covalent bond is formed when two atoms share electrons, typically occurring between nonmetals. In the case of water (H2O), the hydrogen atoms share electrons with the oxygen atom, allowing each hydrogen to achieve a stable electronic configuration. This electron sharing provides the molecule with its characteristic properties, including its polarity.

In contrast, NaCl (sodium chloride) and MgO (magnesium oxide) are examples of ionic bonds, where there is a complete transfer of electrons from one atom to another, resulting in the formation of charged ions that attract each other. Cs2O (cesium oxide) also represents an ionic bond for similar reasons.

Thus, water is the only molecule listed that exemplifies a covalent bond through the sharing of electrons, making it the correct answer in this context.