A storm is brewing means trouble, conflict, or danger is developing. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• When the workers stopped talking, everyone knew a storm was brewing.
• In an HSC essay, A storm is brewing can express this meaning: trouble, conflict, or danger is developing.
• When this expression appears in a passage, connect A storm is brewing with the idea of trouble, conflict, or danger is developing.