The atomic number of magnesium is necessary to determine its valency since it indicates how many electrons are present in the atom. It is simpler to determine the valency directly when you are aware of the group the atom is a part of.

The number of electrons in an atom's outermost shell, also known as the valence shell, is referred to as its valency. Since they are the atoms' loosestly held electrons, it is crucial to study them.
The atomic number of the magnesium atom is 12. It immediately follows sodium Na.
Magnesium is found in group 2 A of the periodic table, which may be found by looking at the periodic table.

Magnesium has an atomic number of 12, which corresponds to the fact that atomic number = number of protons + number of electrons.
The 12 electrons of magnesium are now filled in their orbitals as 1s²2s²2p⁶3s², in accordance with Hund's rule.
As a result, we can observe that Magnesium has two electrons in the first orbital, eight electrons in the second orbital, and two electrons in the final orbital.
Consequently, magnesium's electronic configuration is (2,8,2).
Now that we understand it, every atom strives to adopt the noble gases' steady electrical structure. Noble gases are arranged in a configuration of the form ns²np⁶.
Either exchanging electrons or losing electrons leads to this arrangement.
Magnesium loses two electrons from its outermost 3s shell to adopt the structure of neon, the closest noble gas.
We are all aware that an atom's valency is determined by the number of its outermost electrons. It describes the atom's propensity for combining.
Magnesium thus has a valency of two.

Hence, the correct option is (A).