Copying my answer:
C is the correct answer.
Since the 1930s, the American Presidency has become its central institution. This was not always the case. Prior to FDR, the American Presidency had its strengths but was a co-equal branch with the Legislative and the Judiciary Branch.
FDR reshaped the Presidency during the Great Depression and World War II to make it the "first branch" of American government, or certainly the one that Americans perceive as the most important/powerful