a. Sorry if you picked a. A minus two is not formed by Mg. The magnesium atom has a filled 2s subshell and very little attraction for more electrons. Metals do not gain electrons.
b. Great, Magnesium is in group 2A. It has two outer or valence electrons in a 2s subshell. These two electrons can be removed relatively easily. The cation has a charge of +2 just like calcium.
c. Sorry if you picked -6. There are tremendous repulsions between 6 electrons. Metals like magnesium have relatively weak attractions for the electrons in the neutral atom. They rarely gain electrons to form negative ions.
d. Sorry if you picked +6. The magnesium has a total of 12 electrons. There are two outer electrons and ten "core" or inner electrons. A great deal of energy is required to remove "core" or inner electrons from an atom, because these electrons are closer to the positive nucleus. The nucleus holds the core electrons tightly. No element forms a +6 ion under normal conditions.