Which Electrons Are Responsible for Most Chemical Bonding

Which electrons are responsible for most chemical bonding?

A) outer electron shell because these electrons can be readily exchanged with adjacent atoms
B) innermost electron shell because the electrons can be transferred to the nucleus
C) middle electron shell because they are intermediate in distance between the nucleus and the adjacent atom that bonds with the atom
D) Any electron can exchange with adjacent atoms to form a bond; there is no preference.