Which of the following is NOT a constituent part of Parliament?

The Cabinet Office is not a constituent part of Parliament because it is a government department rather than a legislative body. The primary function of the Cabinet Office is to support the Prime Minister and the Cabinet in the administration of government policy and to coordinate the operations of government departments.

In contrast, the House of Commons is one of the two houses of Parliament and plays a crucial role in shaping legislation and representing constituents. Select committees and public bill committees are also integral parts of Parliament, as they are responsible for scrutinizing government actions, examining proposed legislation, and conducting inquiries. Therefore, the Cabinet Office, while important for the functioning of government, does not form part of the parliamentary structure itself.