Final answer:
Minimalism is the type of opera that became popular in the 20th century.
Step-by-step explanation:
The type of opera that became popular in the 20th century was minimalism. Minimalist opera is characterized by its repetition of simple musical patterns and sparse instrumentation. It emerged as a reaction against the complexity and grandeur of traditional opera.
An example of a minimalist opera is Einstein on the Beach by Philip Glass, which features repetitive musical motifs and a non-linear narrative structure.
Other types of operas listed in the question, such as singspiel (a German form of opera with spoken dialogue) and oratorio (a religious or semi-religious opera), did not gain as much popularity in the 20th century.
Learn more about Types of opera in the 20th century