Which of the following was the most popular form of the new mass culture that emerged during the 1920s?

Vaudeville was a popular theatrical variety show format that included a mix of comedy, music, and performances. While it was widely enjoyed in the early 20th century, its popularity began to decline in the 1920s as movies gained traction, ultimately leading to the waning of vaudeville acts in favor of cinematic experiences.

Horse racing has historically been a popular pastime and betting activity, especially in the early 1900s. However, while it captured the interest of many, it lacked the widespread appeal and accessibility that movies offered, making it a less dominant form of mass culture during the 1920s.

Baseball was indeed America's pastime and enjoyed significant popularity during the 1920s. However, it did not reach the same level of mass cultural impact as movies, which provided a shared experience across diverse populations and became a unifying aspect of American society.

Jazz music flourished during the 1920s, often referred to as the Jazz Age, and played a crucial role in the cultural landscape. Nevertheless, while it influenced music and dance, it was not as universally accessible or impactful as the movie industry, which captured the collective imagination through visual storytelling.