At the threshold of means at the beginning or edge of a new stage. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The nation stood at the threshold of independence.
• In an HSC essay, At the threshold of can express this meaning: at the beginning or edge of a new stage.
• When this expression appears in a passage, connect At the threshold of with the idea of at the beginning or edge of a new stage.