A blot on the landscape means something ugly or unpleasant that spoils an otherwise attractive place. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The abandoned factory is a blot on the landscape near the river.
• In an HSC essay, A blot on the landscape can express this meaning: something ugly or unpleasant that spoils an otherwise attractive place.
• When this expression appears in a passage, connect A blot on the landscape with the idea of something ugly or unpleasant that spoils an otherwise attractive place.