An example of a chemical compound is...

A chemical compound is a substance formed when two or more elements are chemically bonded together, resulting in new properties that are different from the individual elements. In this case, salt, which is scientifically known as sodium chloride (NaCl), is a classic example of a chemical compound. It consists of sodium (Na) and chlorine (Cl) elements that combine in a fixed ratio to form a stable substance.

On the other hand, mercury, copper, and calcium are all elements and do not consist of bonded atoms of different elements. They can exist in their pure forms and are listed in the periodic table as individual elements rather than compounds. Therefore, among the options provided, salt stands out as the only true chemical compound.