The best answer is:
Cl: it takes much more energy to pull an electron away from a valence shell that is nearly full.
This is because chlorine (Cl) has a higher ionization energy than magnesium (Mg) due to its electron configuration. Chlorine has seven valence electrons and is close to having a full outer shell (8 electrons), making it more stable and requiring more energy to remove an electron. In contrast, magnesium has two valence electrons and is further from achieving a full outer shell (8 electrons), so it requires less energy to remove its outer electrons.