Which of these is a popular instrument in bluegrass music?

Bluegrass music is a genre of American roots music that developed in the 1940s in the Appalachian region of the United States. Bluegrass features acoustic stringed instruments and emphasizes the off-beat. The violin (also known as the fiddle), five-string banjo, guitar, mandolin, and upright bass (string bass) are often joined by the resonator guitar (also referred to as a Dobro) and (occasionally) harmonica or Jew's harp. This instrumentation originated in rural dance bands and is the basis on which the earliest bluegrass bands were formed. Traditional bluegrass performers believe the "correct" instrumentation is that used by Bill Monroe's band, the Blue Grass Boys (guitar, mandolin, fiddle, banjo, and bass).

Source: Wikipedia