In the context of the excerpt from "The Open Window," the best response to how the room shapes Nuttel’s assumptions about the strangers who live there is:
He assumes Mrs. Sappleton must either be married or widowed.
This assumption can be influenced by the context of the setting, including the presence of a room that suggests domestic life, and the nature of the conversation that indicates Mrs. Sappleton's status as a central figure in the household. This influences Nuttel's perception of her character even before he meets her.