What is the electronic configuration of magnesium $\left({}_{12}^{24}Mg\right)$?

The correct option is B 2,8,2
The atomic number of magnesium is 12.
The maximum number of electrons that the K, L, M shells can accomodate are 2,8,18 respectively.

Hence, the electronic configuration of magnesium is-
$$\begin{array}{cccc}\text{Shell}& \text{K}& \text{L}& \text{M}\\ \text{No. of electrons}& \text{2}& \text{8}& \text{2}\end{array}$$