The best statement that explains why carbon is present in so many kinds of molecules is because carbon can form up to four covalent bonds and it can form stable bonds with many elements, including itself. This versatility allows carbon to serve as the backbone for an incredibly diverse array of complex and large molecules, including carbohydrates, proteins, lipids, and nucleic acids, which are essential for life. Carbon's ability to form stable chains and complex structures underlies the vast variety of carbon-based molecules found in living organisms and the environment.