The ans is 3f. The lowest principal quantum number (n) for s-orbitals is 1, for p- orbitals is 2, for d-orbitals is 3 and for f-orbitals is 4. Therefore, there can only exist these orbitals: 1s (but not 1p, 1d, 1f); 2s, 2p (but not 2d, 2f); 3s, 3p, 3d (but not 3f); 4s, 4p, 4d, 4f; 5s, 5p ... and so on. Solution The ans is 3f. The lowest principal quantum number (n) for s-orbitals is 1, for p- orbitals is 2, for d-orbitals is 3 and for f-orbitals is 4. Therefore, there can only exist these orbitals: 1s (but not 1p, 1d, 1f); 2s, 2p (but not 2d, 2f); 3s, 3p, 3d (but not 3f); 4s, 4p, 4d, 4f; 5s, 5p ... and so on..