The correct answer is:
A. A chemical bond forms when two atoms transfer or share outer electrons to complete their outer shells.
This description encapsulates the key mechanism of chemical bonding, which typically involves the interaction of valence (outer shell) electrons. Atoms strive for stability, often achieved by having full outer electron shells, and they can accomplish this through ionic bonding (transferring electrons) or covalent bonding (sharing electrons). Options B and C are incorrect because chemical bonds do not involve the fusion of atoms or the transfer of protons.