Arrange the following sublevels in order of
increasing energy: 3d, 2s, 4s, 3p.

To arrange the sublevels in order of increasing energy, we add the principal shell and the maximum number of electrons of the specific sublevel. s has 2 electrons, p has 6 electrons, d has 10 electrons, and f and 14 electrons. in this case, a) 3 + 10 = 13b) 2 + 2 = 4c) 4 + 2 = 6d) 3 + 6 = 9 
The order hence is 2s, 4s, 3p, and 3d