Answer: D.
some substances hold onto electrons tighter than other
substances, so they get the extra electrons.
Explanation: When two substances are in contact by rubbing each other, there will be friction between them. The friction will stimulate what we called - Thermal energy. And what thermal energy does is the disrupt the bonding force or enhance it.
We can therefore conclude that some substances hold onto electrons tighter than other substances, so they get the extra electrons. Because at neutral state of the substance, the number of protons is equal to the number of electrons.