Covalent compounds occur

C only between nonmetals

Explanation:

Nonmetals form covalent bonds by sharing electrons. In the human body, most bonds are covalent, and the body is made up mostly of carbon, hydrogen, oxygen, you get the idea ... which are all nonmetals!

Why not A: Noble gases don't bond, except in rare cases.

Why not B: Metal-nonmetal bonding is called an ionic bond, because electrons are transferred between ions.

Why not D: Metals usually don't bond to each other, and this isn't covalent because they don't share electrons.

I believe the answer is: C

(only between nonmetals).