The two presidents who had the greatest impact on the increase in presidential power between 1930 and 1960 were **Franklin D. Roosevelt and Harry S. Truman**.
Roosevelt's New Deal programs expanded the federal government's role in the economy and society, and he also used his executive power to create new agencies and programs without congressional approval. Truman continued to expand presidential power during the Korean War, and he also used executive privilege to refuse to comply with congressional subpoenas.
The other presidents listed in the answer choices did not have as significant an impact on the increase in presidential power. Calvin Coolidge and Herbert Hoover were both laissez-faire presidents who believed in limited government. Lyndon Johnson and Richard Nixon both expanded presidential power, but they did so during a time when the presidency was already very powerful. Dwight D. Eisenhower and John F. Kennedy both respected the separation of powers and did not seek to expand the presidency's authority.
Therefore, the answer is (C).