Step 1: Identify common salt.
Common salt is chemically known as sodium chloride.
Step 2: Understand the chemical composition.
It is made up of sodium (Na) and chlorine (Cl).
Step 3: Analyze the options.
H$_2$O is water.
KCl is potassium chloride.
CO$_2$ is carbon dioxide.
Step 4: Conclusion.
Hence, the chemical formula of common salt is NaCl.