The most ironic aspect of the setting in "The Open Window" by H.H. Munro (Saki) is:
d. The setting is a "restful country spot," but proves to be unsettling because of the niece's stories.
This irony emerges from the contrast between the serene description of the countryside, which one would expect to be a peaceful retreat, and the unsettling tension created by the niece's unsettling storytelling. The tranquility of the setting is undermined by the shocking tale she weaves, leading to an unexpected twist.