Which type of compounds are formed by carbon? 

Covalent compounds 

The correct answer is option 3. Covalent compounds.

The type of compound formed by carbon is Covalent compounds. Carbon is known for its ability to form strong covalent bonds with other carbon atoms and with atoms of other elements. Covalent compounds involve the sharing of electron pairs between atoms, resulting in the formation of molecules. Carbon forms a wide variety of covalent compounds, including hydrocarbons (such as alkanes, alkenes, and alkynes), organic compounds, and inorganic compounds. Therefore, the correct answer is Covalent compounds.