Folk music is
music with a specified composer
composed by a professional group of musicians
often authored by a group
any music played by a musician with an acoustic guitar
Folk music is often characterized as music that is traditionally passed down through generations, typically reflecting the culture and experiences of a particular community or group. It is usually authored by a group or community rather than a single specified composer, and it often features simple melodies and lyrics that tell stories or convey cultural themes. While it can be played with an acoustic guitar, it is not limited to that instrument. Therefore, the most accurate description from your options would be:
often authored by a group.