So,
Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules.
According to the answer options, the only one that makes sense according to the importance of the covalent bonds that carbon can form is D.
Carbon can form so many compounds because its capacity to form single, double, or triple bonds.
Therefore the correct answer is D.