Merge sort guarantees O(n log n) and quicksort is usually preferred anyway. What is quicksort buying, and what is it risking?

D. A smaller constant and in-place operation, at the risk of O(n²)

Quicksort partitions in place with excellent cache behaviour, where merge sort allocates a second array, a real cost at scale. The price is a quadratic worst case on sorted input with a naive pivot, mitigated by randomised or median-of-three pivots and by falling back to heapsort, which is what introsort does.