Covalent bonds usually form between atoms of elements that share protons valence electrons nuclei oppositely charged ions

If the blank is after 'share' in the original question and Protons, valence electrons, nuclei, and oppositely charged ions are answers, then valence electrons are the answer. Covalent bonds share electrons. a simple rule for protons is "change the protons (number) change the element". When a bond forms the element does not change therefore the protons do not change. 

Answer:

a. energy is absorbed in the reaction.

explanation:

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.

What are the choices?