Final answer:
An Act passed by the Federal Parliament commences after receiving Assent by the Governor-General, following approval by both houses of the legislature.
Step-by-step explanation:
An Act passed by the Federal Parliament of Australia commences after receiving Assent by the Governor-General. This is in line with the conventional legislative process in parliamentary systems where a bill must be formally approved by the head of state after being passed by both houses of the legislature. It is important to note that contrary to option D, the discretion does not rest with the Prime Minister and Cabinet, and option B, 'retrospective' commencement, is not the norm and typically requires explicit provision. Similarly, option C is incorrect as the Act cannot commence before being introduced into parliament, and it must complete the outlined legislative process.