Answer:
D. 3d
Explanation:
This is actually a tricky question, but first, let's eliminate 1s and 2p in the choices because they are lower in energy than 3d and 4s.
Electrons usually go through 4s energy shells before filling in 3d orbital shells but chemists write the electron configuration 3d orbitals before 4s because 4s is usually more stable than 3d due to its orientation.
The usual arrangement of electron orbital is s, p, d, f.
