A) Sarah Lahey

Not Quite...

Sarah Lahey is indeed central to Karl's unravelling, but she wasn't the victim in his nightmare.

Sarah is Karl's detective partner who bears the brunt of his deteriorating mental state. While Karl's obsession with Luke Smith destroys their professional relationship—culminating in Karl violently injuring Sarah on July 29th when he hears "the voice"—she doesn't appear as a victim in the nightmare itself.

The nightmare victim has a more direct connection to Luke Smith's household and appears again in a tragic climax just two days later.