Final answer:
The phrase 'very solitary' from the excerpt best establishes the gothic setting because it conveys a sense of isolation typical of gothic literature.
Step-by-step explanation:
The detail from the excerpt of The Strange Case of Dr. Jekyll and Mr. Hyde that best establishes the gothic setting is "very solitary." This phrase conveys a sense of isolation and emptiness which is characteristic of gothic literature, where settings are often eerie, desolate, and induce a feeling of suspense or foreboding. In contrast, the other details mentioned ("fine dry night," "ballroom floor," and "regular pattern") do not evoke the same dark, mysterious atmosphere typically found in gothic settings.