Final answer:
The significant development during FDR's presidency in terms of the American two-party system was the New Deal coalition supporting the Democratic Party, which led to Democratic dominance for decades.
Step-by-step explanation:
Significant Development in the American Two-Party System During FDR's Presidency
The significant development during Franklin D. Roosevelt's (FDR's) presidency concerning the American two-party system was D) The New Deal coalition that supported the Democratic Party. This coalition transformed the two-party system and resulted in a broad electoral alliance that included various groups and became a dominant force in American politics from the 1930s to the 1960s. The New Deal programs like the Works Progress Administration (WPA) and the Tennessee Valley Authority (TVA) garnered support from diverse groups, leading to the creation of this coalition. Despite various challenges and the emergence of third-party movements, none were successful at the national level. The realignment of political parties during this period was stark, with socioeconomic divides becoming more pronounced between those favoring stability through the Republican Party and those seeking system changes predominantly aligning with the Democratic candidates.
The New Deal coalition was instrumental in building a political machine that led to Democratic dominance in presidential elections and in Congress for several decades. This coalition began to fracture in the late 1950s and early 1960s as social issues took center stage and Southern Democrats started resisting the expansion of federal power into social restructuring, eventually shifting their support to the Republican Party.