Final answer:
The executive branch is the branch of government whose power grew significantly during the 20th century, due to various factors such as the expansion of federal bureaucracies, international prominence, and mass media communication.
Step-by-step explanation:
The branch of government that has seen its power grow during the 20th century is the executive branch. This growth can be attributed to a variety of factors, including the expansion of the federal bureaucracy, increased regulatory power, and the ascendancy of the United States on the international stage. Significant increases in executive power occurred during major events and crises such as the Great Depression, two world wars, the Cold War, and the war on terrorism. The expansion of the bureaucracy in the late 19th and 20th centuries, with institutions like the Federal Reserve Board and the Federal Trade Commission, also played a role in enhancing presidential power. Moreover, technological advancements such as radio, television, and social media have allowed presidents to communicate directly with citizens, further increasing their influence.