Cast a long shadow means to have a lasting and powerful influence. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The war continued to cast a long shadow over the region.
• In an HSC essay, Cast a long shadow can express this meaning: to have a lasting and powerful influence.
• When this expression appears in a passage, connect Cast a long shadow with the idea of to have a lasting and powerful influence.