Carbon most often forms a covalent bond with other atoms. If the bond is with another carbon atom, it is a pure covalent (or nonpolar covalent) bond. If it is with another atom, a polar covalent bond is formed.
The most common oxidation state of carbon is +4 or -4.
Less commonly, carbon forms ionic bonds with other atoms. This occurs when there is a large electronegativity difference between carbon and the other atom.
So, it usually forms covalent bonds. So D.