Final answer:
The rise of radio as a media source was popularized by FDR with his fireside chats, which were radio addresses where he directly spoke to the American people to explain his policies and gain public support.
Step-by-step explanation:
The rise of radio as a media source was popularized by FDR with his speeches known as the fireside chats. These radio addresses were delivered by President Roosevelt and provided an opportunity for him to speak directly to the American people, explaining his policies and gaining their support.
The fireside chats became a powerful tool for FDR to enact his policies and communicate with the public.