# Which of the following is an incorrect designation for an atomic orbital? a) 1s b) 4f c) 3s d) 2d e) 2p

Simply put, the second energy shell, designated by a principal quantum number equal to 2, or $\text{n=2}$, can only hold s and p-orbitals.
For the second energy level, the allowed values for the angular momentum quantum number, or $l$, the quantum number that describes the shape (type) of an orbital that can occupy a certain energy level, are 0, which designate s-orbitals, and 1, which designate p-orbitals.
D-orbitals are allowed starting with the third energy level, or $\text{n=3}$, for which $l$ is equal to 2.