The correct answer is: C. Natural or Synthetic group.
Quartz and Rochelle salt are both piezoelectric materials. Piezoelectric materials are materials that generate an electric charge when they are subjected to mechanical stress, or conversely, that deform when an electric field is applied to them. Quartz is a naturally occurring mineral, while Rochelle salt is a synthetic material. Both materials have been used in a variety of applications, including piezoelectric oscillators, sensors, and actuators.
Option A is incorrect because quartz and Rochelle salt are not both natural materials. Option B is incorrect because quartz and Rochelle salt are not both synthetic materials. Option D is incorrect because quartz and Rochelle salt are not both fiber materials.