The correct statement is:
They have the same number of electron shells.
Nitrogen (N) is in group 15 and period 2, and phosphorus (P) is in group 15 and period 3. They both have the same number of electron shells: nitrogen has 2 electron shells, while phosphorus has 3.
The other statements are incorrect:
- Nitrogen and phosphorus are not classified as halogens (which are in group 17).
- Nitrogen is not inert (it is reactive in certain conditions), and phosphorus is not inert either.
- They do not have the same number of valence electrons; both have 5 valence electrons, which makes the last statement misleading, as it's context-dependent; it could imply they are the same, but more accurately, they're in the same group and have the same amount.
Therefore, the most precise correct response here is about having the same number of electron shells.