* Covalent bonds form when two atoms share electrons to achieve a stable electron configuration (like having a full outer shell).

Let's look at why the other options are incorrect:

* A. Electrons are transferred: This describes an ionic bond, where one atom loses electrons and becomes positively charged, while the other gains electrons and becomes negatively charged.

* C. Protons lost: Protons reside in the nucleus of an atom and are not involved in bonding. Losing protons changes the element itself.

* D. Ions formed: This is a characteristic of ionic bonds, not covalent bonds.

In summary: Covalent bonds are formed by the sharing of electrons between atoms.