Which of these is not a basic principle of radiation protection ?
In addition to the three basic principles of radiation protection (justification, limitation, and optimization), one  can apply the following additional techniques to reduce the radiation burden to the patient:

Collimation of the x-ray beam.
Correct focus-to-skin distance.
Lead apron with thyroid collar.
More radiation-sensitive image receptors.
Justification
Collimation
Limitation
Optimization