When two carbon form a single bond this is called

The answer is: Covalent Bond.

Explanation:

Those bonds which are formed by sharing of electrons present in their outermost shell to attain inert state. When two carbon share electron they give rise to covalent bond. This type of bond formation process can result into long chain of atoms bonded together giving rise to complex structure. The separate branch of chemistry called organic chemistry is focused on the study of properties of compounds made by the C-C bond.