What is the oxidation number of beryllium?

The term "oxidation number" refers to the state of an element that is used to describe the characteristic of how a combination with other elements forms.
The beryllium element's oxidation number must be determined. Knowing the group that an element belongs to allows us to determine its oxidation number. Therefore, the beryllium atom belongs to group 2 and period 2, and since it has an atomic number of 4, it has four electrons. The beryllium will be arranged as follows:

$1s^{2}2s^2$

The valence shell in the configuration must first be identified in order to determine the oxidation state, followed by the valence shell's electron count. Therefore, the beryllium atom has a two-valence shell and two electrons in the second shell.
Now that the atom has these 2 more electrons, it can be reduced to a +2 oxidation state. The most stable oxidation state of these elements is +2, since it belongs to group 2.

Hence, option A is correct.