Which of the following was a communication technology that emerged in the 1920s?

The emergence of the radio in the 1920s marked a significant advancement in communication technology. During this period, radio broadcasting began to develop into a mass medium, allowing information, entertainment, and news to reach a wide audience for the first time. This shift not only changed how people consumed media but also affected social dynamics and culture, as families and communities would gather to listen to broadcasts together.

In contrast, television technology was still in its infancy during the 1920s, with significant advancements occurring later in the 1930s. Jet-propelled airplanes were in the early stages of development but did not impact communication directly. Submarines, while important in naval warfare, were not related to communication technologies in the context of this question. Thus, the radio stands out as the correct answer due to its widespread adoption and impact on society in that decade.