A cog in the machine means a small and unimportant part of a large system. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• He felt like a cog in the machine at the huge company.
• In an HSC essay, A cog in the machine can express this meaning: a small and unimportant part of a large system.
• When this expression appears in a passage, connect A cog in the machine with the idea of a small and unimportant part of a large system.