The correct response would be:
He assumes Mrs. Sappleton must either be married or widowed.
This assumption is based on the context of the room and the social norms of the time. The presence of the open window and the way the young lady speaks about her aunt leads Nuttel to believe that Mrs. Sappleton is a woman of a certain social standing, likely either married or widowed, which shapes his overall perception of the strangers he encounters.