Final answer:
The Magnus Liber Organi was crucial to the development of Western music via the collection of medieval music and the advancement of musical notation systems.
Step-by-step explanation:
The Magnus Liber Organi contained development made possible by the c) collection of medieval music; musical notation. This compilation was instrumental in the development of Western music, laying the foundation for written and theoretical practices that would eventually shape the entire musical tradition of the West. Musical notation allowed for the precise documentation and replication of musical compositions, which in turn facilitated the advancement of polyphony and complex musical structures vital to the evolution of music during the Medieval period.