Which of these best describes the tone of these final words of the speaker concerning the duchess

I would say that the correct answer is ominous.

The poet says the following:

"This grew; I gave commands; Then all smiles stopped together. There she stands As if alive ..."

It is implied that the narrator had his wife, the Duchess, killed for cheating on him. So, I believe the tone is very frightening and scary because of that fact.