The correct answer is D) They are usually pressed onto their shafts. Antifriction bearings, such as ball bearings or roller bearings, are typically pressed onto the shaft to ensure a secure and precise fit. This press-fit installation prevents the bearing from slipping or moving on the shaft, which is critical for the proper functioning and longevity of the bearing. The other options are incorrect because: A) The inner race should be securely fixed to the shaft, not free to turn. B) The outer race should also be securely fixed within the housing, not free to turn. C) Proper alignment of the bearing is crucial for its performance and to prevent excessive wear and premature failure.