Final answer:
Phosphorus is the element most similar to nitrogen with 3 occupied principal energy levels.
Step-by-step explanation:
The element most similar to nitrogen, with 3 occupied principal energy levels, is phosphorus. Phosphorus has the electron configuration of [Ne]3s²3p³, which means it has 3 occupied energy levels, similar to nitrogen. Both nitrogen and phosphorus are in Group 15 of the periodic table, which means they have similar chemical properties.