The correct answer is C) spring. The carbon seal ring of a refrigeration compressor crankshaft mechanical seal is held in position against the stationary ring face by a spring. The spring provides the necessary force to keep the carbon seal ring in contact with the stationary ring face, ensuring a tight seal and preventing leakage from the refrigeration system. The other answer choices are incorrect because: A) a snap ring is used to retain other components, not to provide the force to hold the carbon seal ring; B) a woodruff key is used to transmit torque, not to hold the seal ring in place; and D) a thrust washer is used to support axial loads, not to hold the seal ring against the stationary face.