A shot across the bow means a warning action meant to prevent stronger action later. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The notice was a shot across the bow for careless drivers.
• In an HSC essay, A shot across the bow can express this meaning: a warning action meant to prevent stronger action later.
• When this expression appears in a passage, connect A shot across the bow with the idea of a warning action meant to prevent stronger action later.