Regional boundaries in popular culture began to diminish in the 1920s as a result of

a. the invention of the phonograph

b. NBC radio broadcasting coast to coast

c. the migration of southerners to northern cities

d. regional radio stations being bought out by national station

The answer is "Option b"

Explanation:

The region will be described as a geographical area with certain defining characteristics, and Regional boundaries were locations where these traits or characteristics change of Larger or smaller area, in which radio broadcasting uses the common format of radio. It is a signal type, which can be an analog or digital audio, and other choices were wrong, that can be described as follows:

In option a, It is not correct, because it's time for the invention of radio broadcasting. In option c, It is not correct, because it not moves in the south to north cities direction. In option d, It can't be a national station, that's why it is wrong.