What type of music would be performed in a Mass by monks?

Gregorian Chant is the correct answer because it represents the style of plainchant used in the liturgical setting of the Catholic Church, particularly during the Mass. Monks performed these chants to enhance the spiritual atmosphere of the worship service. These melodies are typically monophonic, meaning they consist of a single melodic line without harmonic accompaniment, which aligns with the more austere and meditative nature of monastic worship.

This type of music is characterized by its free-flowing rhythms and modal scales, reflecting the sacred themes of the texts being sung. Gregorian Chant was specifically composed for the delivery of liturgical texts in a solemn and reverent manner, making it the appropriate choice for performances by monks during religious services.

In contrast, folk songs, popular ballads, and opera represent different genres that cater to varied audiences and contexts outside of the sacred space, often featuring more complex structures, harmonies, and dramatic elements that are not suitable for the solemnity of a Mass.