Assertion (A): The number of absorption transitions is always lower than the number of emission transitions between any two specific energy levels.

Reason (R): Absorption transitions only begin at the lowest energy level and can end at any higher energy level. On the other hand, emission transitions can begin at any higher energy level and conclude at any lower energy level.

Both A and R are true and R is the correct explanation of A.

Beginning from any level $n$ to another upper energy level $m$, only $m-n$ absorption transitions can be made, but since emission from $m$ to $n$ can involve transition between any two levels between $n$ and $m$, ^m−nC_2​ transitions are possible which is always greater than $m-n$. Hence both statements are correct and second one is correct explanation for first.