Which series of the Repertoire International des Sources Musicales Online is focused on arranging music by topic?

The correct answer focuses on Series B of the Repertoire International des Sources Musicales Online (RISM), which is specifically dedicated to arranging music by topic. This series organizes musical works based on thematic content, subject matter, and various categories that allow users to locate music that fits specific genres or themes. This systematic approach enhances research and education by facilitating access to a wide variety of music centered around distinct topics.

The other series within RISM have different focuses; for instance, Series A is more concerned with documenting printed music for which a significant collection exists, while Series C is largely centered on manuscripts and archival materials. Series D contains musical sources related to the field of theater. Therefore, Series B stands out for its unique purpose of organizing content thematically, thus making it a valuable resource for educators, researchers, and musicians looking to explore music within specific topics or areas of interest.