In which main energy level does the 'd' sublevel first appear? k (first main energy level) l (second main energy level) m (third main energy level) n (fourth main energy level)

M (third main energy level)

Explanation:

The third main energy level bears the first appearance of the 'd' sublevel. The principal quantum number(n) depicts the main energy levels in which an orbital is located. It takes values of n=1,2,3,4,5 and it can be represented by the shells k,l,m,n

The subshells in these main orbitals are represented by s,p,d and f. For the K shell, the principal quantum number is m and its sublevel notations are s,p and d. This is where the d-sublevel first appears.

with this one i know the answer in craters one reason i looked it up and i learned about it there is my evidence

frequency, z, and the collision density, z, in carbon dioxide (co2), r 180 pm at 30 °c and 120 kpa