What is the most likely reason that nonmetals are electrical insulators?

since valence shells are mostly empty in nonmetal atoms, the atoms attract and hold any electrons they can in order to fill their valence shells.

since valence shells in nonmetal atoms are almost full, the atoms attract electrons and hold them tightly to fill their valence shells.

since all nonmetals have valence shells that are full, they hold electrons tightly to avoid changing.

since all nonmetals have valence shells that are completely empty, they do not attract or release any electrons.

The reason nonmetals are electrical insulator is since valence shells in nonmetal atoms are almost full, the atoms attract electrons and hold them tightly to fill their valence. The reason why it acts as an insulator is the high ionization energy it has. It prevents any electron from leaving an atom easily even when an electric field is applied. 

a. energy is absorbed in the reaction.

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.