One reason for the growing power of the president in foreign affairs since the early 20th century is:

The correct choice highlights that the United States' emergence as a global power has significantly contributed to the growth of presidential authority in foreign affairs. As the U.S. expanded its influence internationally in the 20th century, the need for decisive and rapid executive action in foreign policy became paramount. Presidents have often taken on a more prominent role in managing international relations, crises, and conflicts, reflecting the country's status as a leader on the world stage.

This increased responsibility has fostered an environment where the executive branch could operate with greater autonomy, often in the context of national security, diplomacy, and military engagements. The complexity and scope of modern global issues necessitate a more active presidential role to respond swiftly, allowing the president to make strategic decisions that align with U.S. interests abroad.

While there are other factors that have played a role in enhancing presidential power, such as legislation influencing Congressional influence, public support for military engagement, or budgetary control, the overarching theme of the U.S. becoming a global power serves as a primary driver for the growth of presidential authority in the realm of foreign policy. This transformation underscores the dynamic nature of the presidency and its relationship with international affairs in a changing world.