Final answer:
From the 1830s to the 1870s, the most popular form of musical theater in the United States was melodrama, known for its clear moral divisions and dramatic special effects.
Step-by-step explanation:
The most popular form of musical theater in the United States from the 1830s to the 1870s was the melodrama. This exaggerated style of morality play was particularly known for its stock characters embodying good and evil, as well as its dramatic special effects and music that heightened the emotional impact of the play. In contrast to realism that dominated much of the 20th century, and the musical comedies that started emerging around the mid-19th century, melodrama was the quintessential format for live theater at this time, reflecting Victorian standards of thought and morality through its clear distinctions between hero and villain, along with grandiose and episodic narratives.