Standard quick sort with the first element as pivot has worst case

When the partition always produces a split of size 1 and n−1 (e.g., the input is already sorted and we pivot on the first element), the recurrence becomes T(n) = T(n−1) + Θ(n) which solves to Θ(n²). This is bubble-sort territory. Best case (balanced splits) is Θ(n log n) the other options are too low.