Answer : Option C) The distribution of electrons in different shells.
Explanation : According to the attached image, the model of the sodium atom represents that the sodium atom has 12 neutrons and 11 protons in its nucleus. Also, there are 11 electrons which are distributed in different shells. The first shell contains 2 electrons, second has 8 electrons whereas third shell has only one electron it. Which defines the valency of the sodium, as 1. As it donates it's last electron to achieve an octet state.
