A skeleton in the cupboard means a hidden and embarrassing secret from the past. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The scandal revealed a skeleton in the cupboard.
• In an HSC essay, A skeleton in the cupboard can express this meaning: a hidden and embarrassing secret from the past.
• When this expression appears in a passage, connect A skeleton in the cupboard with the idea of a hidden and embarrassing secret from the past.