All of the alkali earth metals, group 2, have two valence electrons. which of these would represent the oxidation number of the alkali earth metals such as magnesium and calcium?

Since Group 2 alkali earth metals have 2 valence electrons, they tend to lose those 2 when forming ionic bonds. And the Loss of Electrons = Oxidation (L.E.O. for short). Therefore this group, including Mg and Ca, have an oxidation of [+2].
So the correct answer is C) +2

The formula is (nh4)2co3