Final answer:
The material that does not belong in the group used for making the gentle and peaceful tones of Korean music is Plastic, as Korean instruments are traditionally made from natural materials like bamboo, wood, and clay.
Step-by-step explanation:
The material that does not belong in the group of natural materials used to create the gentle and peaceful tones of Korean music is C. Plastic. Traditional Korean musical instruments are often made from materials like bamboo, wood, and clay, which are abundant in Asia and provide a particular resonance and timbre to the music. These natural materials have been used historically to construct instruments because they are biodegradable and work in harmony with the environment. Unlike these natural materials, plastic is a synthetic material, which is why it does not belong in this group.