The correct option is B.
Chlorine has seven electrons in its outermost shell, to attain the octet structure, it usually accept electrons from metals. In so doing, it attains an oxidation number of -1 and become an anion. An anion refers to a charged particle that carries a negative charge while a cation refers to a charged particle that carries a positive charge. Chlorine anion has a charge of - 1 because chlorine accept one electron from another element.