At a moment's notice means with very little warning or preparation time. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The nurse was ready to help at a moment's notice.
• In an HSC essay, At a moment's notice can express this meaning: with very little warning or preparation time.
• When this expression appears in a passage, connect At a moment's notice with the idea of with very little warning or preparation time.