Which type of music structure is characterized by independent melodies sung simultaneously?

The correct answer is polyphonic, which refers to a musical structure where multiple independent melodies are sung or played simultaneously. This style is characterized by the weaving together of different melodic lines, creating a rich and complex texture. Each melody holds its own significance and is developed independently, allowing for a harmonious interaction between them.

Polyphonic music can be seen in various genres, particularly in classical compositions, such as those by J.S. Bach, where counterpoint—the relationship between two or more independent melodies—plays a critical role. This contrasts with other musical structures, such as monophonic, where there is a single melody without any accompanying harmony, or homophonic, where there is a primary melody supported by chords or harmonies. Atonal music, while it may employ multiple lines, does not focus on the independence of melodies in the same way polyphony does.