Final answer:
The unique instrument to country music and not typically used in folk or bluegrass is the pedal steel guitar (Option C).
Step-by-step explanation:
The instrument that is widely used in country music and almost never used in folk or bluegrass is the pedal steel guitar (Option C). While the mandolin, banjo, and fiddle are common to all three genres, the pedal steel guitar has a distinctive sound that has become synonymous with country music.
It's an electric instrument with a set of foot pedals and knee levers that change the pitch of the strings, providing the characteristic gliding sound associated with country music.
In contrast, folk music often relies on lightweight, homemade instruments or a cappella singing, and bluegrass music is known for its acoustic string instruments and emphasis on virtuosic instrumental solos.
Over time, these genres have developed their own distinctive sounds, influenced by the availability, cultural significance, and acoustic properties of the instruments traditionally used in their music.