Nuttel's assumptions about the strangers are shaped by the decor and atmosphere of the room he enters, which reflects a certain order and expectation. Therefore, the most fitting response is:
He assumes Mrs. Sappleton must either be married or widowed.
This assumption arises from the societal norms and expectations reflected in the room's furnishings and the presence of the young lady, which indicate a sense of domesticity that would typically accompany a woman in those marital statuses.