Valence electron(s) are the electron(s) located on the outermost shell of an atom. Valency is simply defined as the combining power of an atom.
Chlorine (Cl) atom has 7 valence electron. This implies that Cl needs just one electron to complete it's octet configuration. It will be difficult for Cl to lose any of it's valence electron(s). Cl can either gain or share 1 electron to become stable.
Thus, considering the options given in the question above, option C gives the correct answer to the question.
