In the context of Saki's short story "The Open Window," the room's shape and decor lead Nuttel to make specific assumptions about the people who inhabit the house. The most applicable choice based on the room's characteristics and the societal norms of the time would be:
A) He assumes Mrs. Sappleton must either be married or widowed.
This assumption is influenced by the traditional expectations of women during that period, where a room decorated with signs of a domestic presence (like photos or specific furnishings) implies the marital status of the occupant. Nuttel's expectations are shaped by the surroundings, leading him to make conclusions about Mrs. Sappleton based on the atmosphere of the room.