Step-by-step explanation:
The Aufbau Principle explains to us how electrons fill low energy orbitals (the one that are closer to the nucleus) before they fill higher energy orbitals. To make the electron configuration we usually use this diagram.
We have to follow the arrows, we start with the orbitals in 1s, then 2s, 2p, 3s, 3p, 4s and so on.
If we consider our question, after we reach the orbital 3s we continue in the next arrow. So after 3s we have to fill 3p.
Answer: a. 3p