Final answer:
The genre that was not a major Hollywood genre during the 1920s is Sci-Fi. While Musicals, Westerns, and Horror were popular, science fiction films did not become significant until later decades.
Step-by-step explanation:
The genre that was not a major Hollywood genre during the 1920s is D) Sci-Fi. While Musicals, Westerns, and Horror were established film genres during this era, science fiction had not yet gained significant ground as a popular genre in Hollywood.
The 1920s was a time when Hollywood became the epicenter of the film industry, with genres like Westerns and Horror gaining popularity. The classic horror film The Cabinet of Dr. Caligari, released in 1920, is an example of the genre's early success.
Musicals would rise to prominence in the 1950s with titles like Guys and Dolls and West Side Story. On the other hand, notable science fiction films like Invasion of the Body Snatchers would only emerge in the 1950s, suggesting that Sci-Fi as a significant genre developed later than the other options provided.