Answer: 2
Notes: Valency is number of electrons gain or loss to achieve noble gas configuration I.e. fully filled octet. Magnesium (Atomic number = 12) has electronic configuration 2, 8, 2. Since magnesium has 2 electrons in its outermost shell, hence it's valency is 2, i.e., it will lose it’s 2 electrons to achieve noble gas configuration (2,8).
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