The correct response is:
Cl; it takes much more energy to pull an electron away from a valence shell that is nearly full.
Chlorine has a higher ionization energy than magnesium because chlorine has seven valence electrons and is close to having a full octet (8 electrons), making it more energetically unfavorable to remove an electron. In contrast, magnesium has two valence electrons and is located further away from achieving a stable electron configuration. Therefore, it takes more energy to remove an electron from chlorine than from magnesium.