Answer:
B. The atom gains 1 electron, to make a total of 18 electrons.
Explanation:
Chlorine is the 17th element in the periodic table, so it has atomic number 17:
Z = 17
This means that a neutral atom of chlorine has 17 protons and 17 electrons.
When a chlorine atom gains 1 electron, its electric charge (initially zero) becomes -1, since the electron has negative charge of -1 (in elemntary charge units). This also means that the number of electrons in the ion is now
17 + 1 = 18
So the correct answer is
B. The atom gains 1 electron, to make a total of 18 electrons