Final answer:
The National Security Council is the unit that connects the president's principal foreign and military policy advisors, providing guidance on security issues and overseeing intelligence agencies.
Step-by-step explanation:
The unit that links the president's key foreign and military policy advisors is the National Security Council (NSC). The National Security Act of 1947 established the NSC to advise the president on security issues and oversee the Central Intelligence Agency (CIA). It includes senior-level staff from multiple foreign policy agencies and is typically headed by the national security advisor, who serves as the president's top foreign policy advisor. In addition to the NSC and the CIA, the director of national intelligence oversees the entire intelligence community, and the Joint Chiefs of Staff, who consist of senior military leaders, also serve as key military advisors to the president.