What does series C of the Repertoire International des Sources Musicales Online index?

The correct answer focuses on Series C of the Repertoire International des Sources Musicales (RISM) Online, which specifically indexes music libraries, private collections, and archives from across the globe. This series provides access to information about a vast array of musical sources that are held in various institutions and personal collections worldwide, facilitating research into music history, performance practice, and the preservation of musical heritage.

By exploring these resources, researchers and musicians can uncover a wealth of information about different types of music, composers, and historical contexts. This resource is crucial for musicologists, educators, and performers interested in accessing rare or unique materials.

The other options, while related to music resources, do not specifically align with the focus of Series C. Public music collections, music theory resources, and historical music documentation serve different purposes and therefore are not categorized within Series C of RISM.