At pains to means making a careful or serious effort to do something. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The author was at pains to explain every term.
• In an HSC essay, At pains to can express this meaning: making a careful or serious effort to do something.
• When this expression appears in a passage, connect At pains to with the idea of making a careful or serious effort to do something.