Answer: The correct answer is Gain of electron.
A neutral atom is the one, which does not carry any charge. This means that the number of electrons ( negative electric charge) and protons ( positive electric charge) are equal in this atom. When it gains an electron ( a subatomic particle with negative electric charge), it becomes negatively charged because the number of negative charge becomes more.
Thus, a neutral atom can be negatively charged by gaining an electron.
