In "The Open Window," Nuttel's assumptions about the strangers he encounters are influenced by the room's appearance and decor. The way the room is furnished, with the open window and the presence of the young lady, suggests a sense of openness and vulnerability that shapes Nuttel's perceptions. He assumes Mrs. Sappleton must either be married or widowed because the room's setup indicates that it is designed for a family or a woman likely awaiting someone. This assumption aligns with the social norms of the time regarding women's roles and their living arrangements.
So, the correct answer is: He assumes Mrs. Sappleton must either be married or widowed.