To predict how many electrons an element can gain or lose, we can be guided by the group to which it belongs in the periodic table. Chlorine belongs to group VII of the periodic table. This means that it has 7 valence electrons.
According to the octet rule, for an atom to be stable it must have 8 electrons in its last energy level, this means that chlorine can gain one electron to comply with the octet rule.
So, the answer will be: a.1