Blow the lid off means to expose a hidden scandal, secret, or truth. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The investigation helped blow the lid off the corruption.
• In an HSC essay, Blow the lid off can express this meaning: to expose a hidden scandal, secret, or truth.
• When this expression appears in a passage, connect Blow the lid off with the idea of to expose a hidden scandal, secret, or truth.