Which of these orbital designations are invalid? 13.2 a. 4s b. 3f c. 2d d. 3d?

In quantum mechanics, there are 4 letters used to designate the orbitals. These are s, p, d, and f. S orbitals can accommodate 2 electrons, p can accommodate 6, d can accommodate 10, and lastly, f can accommodate 14 electrons. 

These orbitals are filled-up based on the energy levels and the configurations are usually,
      1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ ...

From the given, only the second choice is not possible. The answer is letter B.