Each cabinet department is headed by a Secretary who sits on the President's Cabinet, with the exception of the Justice Department, headed by which of the following?

The Justice Department is uniquely headed by the Attorney General, who is a member of the President's Cabinet. The Attorney General is responsible for overseeing federal law enforcement and representing the United States in legal matters. This position is significant as it ensures that the enforcement of law and administration of justice aligns with federal interests.

In contrast to the Attorney General, the other options do not accurately represent the head of the Justice Department. A Supreme Court justice serves on the highest court in the federal judiciary but does not lead a cabinet department. The term "Attorney-at-law" is a general term for anyone who is qualified to practice law, but it does not refer to a specific position or title within the government structure. Similarly, "Secretary of Justice" is not a recognized title within the U.S. federal cabinet; instead, the title for the head of the Justice Department is specifically "Attorney General." Thus, this clarifies why the Attorney General is the correct answer.