Final answer:
The statement that a key fits into a slot in a shaft and a mating part to prevent the part from turning on the shaft is true. This is a common engineering practice to transfer torque and maintain component alignment.
Step-by-step explanation:
A key that fits into a slot cut in a shaft and a mating part does indeed keep the part from turning on the shaft; hence, the answer is A) True. This is a principle used in mechanical engineering to transfer torque between components and ensure that parts maintain their relative positions during operation. Keys, along with keyways, are used as a failsafe to prevent motion where it's not wanted, often seen in machinery and engines.