The correct answer is: C. conducting and non-magnetic material.
Eddy current damping is a type of damping that uses eddy currents to dissipate energy. Eddy currents are induced in a conducting material by a changing magnetic field. The eddy currents then flow in a circular motion, creating a magnetic field of their own. This magnetic field opposes the original magnetic field, which causes the eddy currents to dissipate energy.
The disc of an instrument using eddy current damping should be made of a conducting material so that eddy currents can be induced in it. However, the disc should not be made of a magnetic material, because the magnetic field created by the eddy currents would interact with the magnetic field of the instrument, which could cause problems.
Option A is incorrect because the disc should be made of a conducting material, not a magnetic material.
Option B is incorrect because the disc should be made of a conducting material, not a non-conducting material.
Option D is incorrect because the disc should be made of a conducting and non-magnetic material.