The correct answer is D) They are usually pressed on to their shafts. Antifriction bearings, such as ball bearings or roller bearings, are typically pressed onto the shaft to ensure a secure fit and proper alignment. This is crucial for the bearing to function correctly and support the rotating equipment, such as a pump. The inner race of the bearing is designed to be tightly fitted to the shaft, while the outer race should be able to rotate freely within its housing. The other answer choices are incorrect because: A) The inner race should be tightly fitted, not free to turn on the shaft. B) The outer race should be free to turn in its housing, not the other way around. C) Proper alignment of the bearing is a critical factor for its installation and operation.