Who decides on how much x ray energy is safe

The International Commission on Radiological Protection (ICRP) and the National Council on Radiation Protection and Measurements (NCRP). Both authorities issue recommendations to regulatory and advisory agencies on the fundamental principles on which appropriate radiological protection can be based

Answer sample (c); // would be best because the percentage of the energy in an incident wave that remains in a reflected wave from this material is the smallest;

+10 that is always the common denominator