Final answer:
The Grand Ole Opry, which began broadcasting in the 1920s from WSM in Nashville, is the most important radio show in the history of country music and remains the longest-running live broadcast radio show in the U.S.
Step-by-step explanation:
The radio show that went on the air in the 1920s and became the most important radio show in the history of country music is the Grand Ole Opry. Originating from WSM in Nashville, it began broadcasting live music and rapidly established itself as a cornerstone for country music enthusiasts. To this day, the Grand Ole Opry continues to be aired every week, making it the longest-running live broadcast radio show in U.S. history. The show played an instrumental role in shaping American culture and rallying listeners from all over the nation, which helped create a unified national identity.