Which work serves as an authoritative reference for jazz, rock, and hip-hop?

The Encyclopedia of Popular Music is considered an authoritative reference for jazz, rock, and hip-hop due to its comprehensive coverage of various genres within popular music. It provides detailed information about artists, albums, songs, and the cultural contexts surrounding these music styles. This resource is particularly valuable for understanding the evolution and impact of popular music genres, including their historical significance and influence on society.

The other options, while valuable, have different focuses. The New Grove Dictionary of Music and Musicians is more extensive in classical music and music theory, while The Music Index Online provides a searchable database for music literature and might not focus as specifically on popular genres. The Garland Encyclopedia of World Music covers music traditions around the globe but is not limited to the popular music genres of jazz, rock, and hip-hop. Hence, The Encyclopedia of Popular Music stands out as the most relevant for these specific areas of interest.