The following statement is the most accurate interpretation of John's reaction: "Her husband's laughter is condescending as he thinks of her observations as foolish" (C).
Based on the context and the general tone of The Yellow Wallpaper, we can assume that the protagonist's husband was being condescending because he has a patronizing behaviour throughout the story. Indeed, he takes his wife to this summer house in order for her to recover from a "temporary nervous depression – a slight hysterical tendency" and prevents her from working because he believes this is the best way she will get better.
The author of the book was denouncing the often unfair dynamics in marriages at the time (late 19th century). When they arrive in the new house, the wife is suspicious of its weird prison-like characteristics, but her husband ignores her observations.