Respuesta :
The third option is correct as it has a total of 12 electrons, with 2 in the valence shell.
The diagram that represent the correct distribution of electrons in electrons shell of Magnesium atom is Diagram 3
explanation
Magnesium has three electron shell, that is 2.8.2 whereby the first electron shell hold a maximum of two electrons, the second one hold a maximum of eights electrons while the rest two electrons(valence electrons) are distributed in the 3rd electron shell.