A disadvantage of Heapsort is:

It is not stable (i.e., records with equal keys might not remain in the same order after sorting)
  • It needs a lot of auxilliary storage
  • Its average case running time is O(n^2)
  • None of these answers

Heapsort does not need auxilliary storage.

Heapsort runs in \Theta(n \log n) time.

Equal-valued records might be in different sides of the heap, and can get out of relative order.