Which of the following describes Pérotin's contribution to organum?

Pérotin's contribution to organum is significant because he was one of the early composers who expanded this form from two voices to three, four, or even more voices. This development marked a crucial evolution in Western music during the medieval period, notably during the Notre Dame school of polyphony. His works exemplify a shift from simple two-voiced organum to a more intricate texture involving multiple independent melodic lines. This complex layering of voices enabled richer harmonies and a more dynamic musical experience, setting a foundation for future developments in polyphonic music and significantly influencing composers that followed.

In contrast, simplifying the structure to create monophonic works, focusing solely on Gregorian chant, or primarily writing solo instrumental music does not align with Pérotin's innovative approach and contributions to the development of polyphonic styles during his time.