Final answer:
A pi bond is formed from the side-to-side overlap of two pi orbitals, not from sigma orbitals or a combination of sigma and pi orbitals. Delta bonds do not play a role in the formation of pi bonds.
Step-by-step explanation:
According to valence bond theory, a pi bond between two atoms would result from the overlap of two pi orbitals. A pi bond is formed when the lobes of the p orbitals or d orbitals extend above and below the internuclear axis and overlap. This is different from a sigma bond, which is formed from the head-on overlap of orbitals, such as s orbitals or hybrid orbitals, and is the first bond formed between any two atoms during covalent bonding. Delta bonds are a higher order bonding type that involves the overlap of d orbitals, but they are not involved in the formation of pi bonds.