2024 AMC 10A Problem 6

Each adjacent swap changes the relative order of exactly one pair of letters

In the fully reversed string every pair of letters is out of order; count the pairs with $\binom{6}{2}$

Reversing all six letters flips the relative order of every pair, so all $\binom{6}{2} = 15$ pairs end up inverted. Each adjacent swap fixes exactly one inversion. So we need at least $15$ swaps, and bubbling each letter into place hits $15$ exactly. Therefore, the answer is D.