Which of the following is a Cabinet-level position?

The Secretary of Defense is a Cabinet-level position, making it the correct choice. The Cabinet consists of the heads of the executive departments of the federal government, and the Secretary of Defense oversees the Department of Defense, which is responsible for coordinating and supervising all agencies and functions related to national security and the military.

In contrast, the Chief of Staff, while an important role in the President's administration, does not hold a Cabinet-level position; it is considered more of a staff role. The Speaker of the House is a leadership position within the legislative branch of government and does not serve in the Cabinet. The Chairman of the Joint Chiefs, while a critical military position, is not part of the Cabinet either; it is a role that leads the military’s highest-ranking officers and advises the President, but it does not have the same status as Cabinet members.