c. share electrons
In covalent compounds, atoms share electrons to achieve a full outer electron energy level through the sharing of electrons in covalent bonds.
In ionic compounds, atoms transfer electrons to form ions, but these ions are held together by electrostatic attraction, which is a result of the sharing of electrons between positively and negatively charged ions. So, even in ionic compounds, there is still a form of electron sharing involved in the overall structure of the compound.