A hair's breadth means a very small distance, difference, or margin. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The cyclist missed the bus by a hair's breadth.
• In an HSC essay, A hair's breadth can express this meaning: a very small distance, difference, or margin.
• When this expression appears in a passage, connect A hair's breadth with the idea of a very small distance, difference, or margin.