Which of the following best explains why a magnesium atom most commonly forms an ion with a 2+ charge?
a) Being in Group 2, magnesium automatically loses its two valence electrons and forms a 2+ ion.
b) Removing one electron leaves a half-filled 3s subshell, which is assumed to be especially stable.
c) Losing two electrons gives Mg the noble-gas configuration of Ne, which is highly stable.
d) The second ionization energy of Mg is lower than the first, so it is favorable to remove exactly two electrons.