Which of the following is not an example of a covalent bond?

Carbon atoms create the materials carbide, diamond, and graphite.
 

Each carbon atom in a diamond is joined by four other carbon atoms to form a hard three-dimensional structure. 

Each carbon atom in graphite is connected to three more carbon atoms in the same plane, forming a hexagonal arrangement. A double bond is one of these bonds.
 

The hexagonal arrangement creates the graphite structure. There are three covalent bonds in carbide. Two of these three covalent bonds are $\mathrm{\pi }$ bonds.

NaCl is an ionic compound which is made up of sodium(Na⁺) and chlorine(Cl^-) ions. So, it is an example of an ionic bond, not a covalent bond.