What organization is known for its three series focusing on music sources since 1952?

The Repertoire International des Sources Musicales (RISM) is the correct answer because it is a comprehensive project established in 1952 that aims to document the world's musical sources, including manuscripts, printed music, and other materials essential for music research. RISM has developed three main series: the series for manuscripts, the series for printed music, and the series for music archives, which collectively serve as critical resources for musicologists, researchers, and anyone interested in the history of music. The organization focuses on cataloging and providing access to these musical sources, making it a vital player in the preservation and study of musical heritage.

In comparison, Oxford Music Online is primarily an online platform that offers a variety of musical reference materials, but it does not specifically focus on cataloging music sources like RISM. The International Index of Music Periodicals is geared towards indexing articles in music journals but does not serve the purpose of documenting musical sources. Lastly, The Encyclopedia of Popular Music is a reference work focused specifically on popular music, rather than encompassing a broader range of music sources.