is the correct configuration for a neutral sodium atom.

A neutral sodium atom contains eleven electrons, so you can eliminate  and  which contain twelve and ten electrons, respectively.

Next,  can be eliminated because of the Aufbau principal, which states that electrons are placed into orbitals from the lowest to highest energy. In this answer choice, the  (lower energy) orbital is not completely filled, containing only five out of the possible six possible electron that it can hold. This orbital must be filled completely before electrons can be placed in the  (higher energy) orbital.

 is incorrect because there are seven electrons in the  orbital. This is impossible, as p orbitals hold a maximum of six electrons.

The noble gas electron configuration for fluorine, , in its ground state is  . We can see from this configuration that there are seven valence electron in its outer shell of  

When an element gains or loses charge, becoming a cation or an anion, it is either gaining or loosing electrons. Anions (negatively charged ions) have gained electrons and cations (positively charged ions) have lost electrons. 

The  ion has gained one electron, increasing its total valence electrons by one. The new noble gas electron configuration for the ion will be , with eight valence electrons. The ion is now isoelectronic to the noble gas neon, and satisfies the octet rule.

To write the noble gas configuration of an element, choose the noble gas in the row above the element to put in brackets. For sulfur, that is the noble gas neon, .

Then, you write the rest of the configuration beginning on the same row as the given element. Sulfur is in the third row, or period, which means that it must start with the orbital. Continuing to fill the orbitals, we get the configuration .

Writing electron configuration this way can save time, and easily tells us how many valence electrons an element has. Sulfur has six valence electrons, shown by the number of electrons in the third energy level.

In its ground state, phosphorous has five valence electron.

We can determine the number of valence electrons by examining the ground state electron configuration of an element. The ground state electron configuration of phosphorous is:

From this configuration we can see that the outermost shell is . In its outer shell there are two electrons in the  subshell and three electrons in the  subshell, giving us a total of five valence electrons. Keep in mind that the different subshells correspond to different orbitals in the same energy level. When determining valence electrons, all electrons in the highest energy level must be included.

A noble gas electron configuration is achieved when an atom has an octet electron configuration, indicating its most stable state. For example, sulfur (S), at its ground state, has 6 valence electrons. When it gains two electrons (-2 charge), it has eight electrons, fulfilling the octet.

All of the answer choices, except , have octet/noble gas electron configurations. Phosphorus (P), at its ground state, has 5 valence electrons. A -2 charge will create an ion with 7, not 8, electrons.