Fill the bill means to be suitable for a need or purpose. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• This simple explanation will fill the bill.
• In an HSC essay, Fill the bill can express this meaning: to be suitable for a need or purpose.
• When this expression appears in a passage, connect Fill the bill with the idea of to be suitable for a need or purpose.