An atom which gains electrons becomes negatively charged and an atom that donates electrons become positively charged.
Therefore, when we rub a silver coin against a wool cloth then cloth becomes positively charged this means that the cloth has lost electrons and the silver coin has gained electrons.
Thus, we can conclude that the statement silver coin gained electrons is true.
It is given that on rubbing silver coin against woolen cloth, the cloth becomes positively charged. A cloth getting positively charged means that it has lost electrons as electrons are negatively charged particles.
On the other hand, a silver coin will gain this negatively charged electron from the wool cloth and thus become negatively charged as in the process of rubbing the static electricity is produced and if one substance get positively charged, the other one will become negatively charged.
It's not erosion, so that is out of the answer choices. And I doubt it is crystallization, so that is why I think it is A and D. (correct me if I am wrong.)
