Chlorine has seven electrons in its valence shell. it accepts an electron to complete its octet and becomes a chlorine ion ( what is the nature of this chlorine ion? a) cation b) anion c) metal ion

As chlorine has seven electrons in its outer most shell so to complete its octet it has to gain an electron and when it gain an electron it will become an anion that is negatively charged
 so in my opinion and what a conclude is that the option B is correct for the above statement
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