Final answer:
The advisory unit that has declined as a significant resource for the president is the Cabinet (as a whole). The rise of specialized advisors within the White House and the Executive Office of the President has led to this relative decline in the Cabinet's advisory role.
Step-by-step explanation:
The presidential advisory unit that has declined significantly as an advisory resource for the president in the twentieth century is E. the Cabinet (as a whole). Over time, the roles and influences of various presidential advisors and councils have evolved. The Executive Office of the President, which includes the Office of Management and Budget, Council of Economic Advisers, and the National Security Council, along with the President's White House Office, have played increasingly integral roles in shaping policy and advising the president. The Cabinet, which was once the principal advisory body to the president, has seen a relative decline in its advisory capacity in the modern era due to the rise of other specialized advisors and councils within the White House and the Executive Office.
Moreover, the White House staff and the nearby Old Executive Office Building staff have grown significantly in numbers, reflecting a trend toward a more complex advisory structure surrounding the president. Noting this, although the Cabinet continues to hold significance as a group of senior officials heading executive departments, it has become less central as a source of daily presidential counsel compared to specialized advisors, such as the national security advisor and chief of staff, who often have more regular interaction with the president particularly on matters of immediate importance.