At the height of means during the most intense or advanced stage of something. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The city was silent at the height of the storm.
• In an HSC essay, At the height of can express this meaning: during the most intense or advanced stage of something.
• When this expression appears in a passage, connect At the height of with the idea of during the most intense or advanced stage of something.