Which of the following began in the late nineteenth century as an experimental form and had developed into a medium of mass culture by the 1930s?

Photography emerged earlier in the 19th century, with its invention traced back to the 1830s. While it became a widely practiced art form and a means of documentation, it did not develop into a medium of mass culture in the same way that motion pictures did, particularly by the 1930s.

Modern dance began to gain popularity in the late 19th and early 20th centuries but remained primarily an artistic expression rather than a mass cultural phenomenon. By the 1930s, it was still largely confined to theatrical performances and did not achieve the widespread appeal that motion pictures did.

Theater has a long history dating back to ancient times. While it experienced significant growth and evolution, particularly in the 20th century, it did not transform into a mass culture medium in the way that motion pictures did during the 1930s, as it remained more localized and dependent on live audiences.

Rock and roll music emerged in the late 1940s and 1950s, well after the time frame specified in the question. Although it became a significant cultural force, it could not have been a form of mass culture by the 1930s as it had yet to be developed and popularized.