Which of the following represents a category of RISM publications?

The category of RISM publications that is accurately represented is indexes of music libraries. RISM, which stands for Répertoire International des Sources Musicales, focuses on the documentation and cataloging of musical sources. The organization strives to provide comprehensive information about music manuscripts and printed music in libraries around the world.

Indexes of music libraries are essential tools that help researchers, musicians, and musicologists locate musical works, understand the availability of sources, and access historical music literature. By compiling this data, RISM facilitates scholarly research and fosters an appreciation for the vast array of music literature available, thus playing a critical role in preserving and disseminating musical heritage.

Historical documentation, periodicals, and musical composition guidelines, while related to music, do not align with RISM’s primary focus on cataloging and indexing sources. Historical documentation may concern broader narratives and context within music history, periodicals are typically publications of ongoing articles and studies in various fields, and musical composition guidelines pertain to practices and theory in writing music, which fall outside the scope of RISM's mission.