Final answer:
Option (a) 1p is not a possible sublevel since the principal quantum number n=1 only allows for an s sublevel, and the p sublevels begin with n=2.
Step-by-step explanation:
The question asks which of the listed sublevels is not possible. The possible sublevels for atomic orbitals are determined by the quantum numbers associated with an atom's electron configurations. According to the principal quantum number (n) and the azimuthal quantum number (l), the sublevel designation follows the sequence s (l=0), p (l=1), d (l=2), and f (l=3).
For option (a) 1p, the principal quantum number n=1 does not allow for a p sublevel since the p sublevel starts from n=2. Therefore, 1p is a not possible sublevel. The other options (b) 2s, (c) 3d, and (d) 4f are allowed as the principal quantum number n is sufficient to include s, d, and f sublevels respectively.
Therefore, the answer to the question is that option (a) 1p is not a possible sublevel because the principal quantum number n=1 only has an s sublevel (1s), and the p sublevels begin with n=2 as in 2p.