Final answer:
Protective aprons are required for radiation levels at or exceeding 5 rad/hr, equivalent to 50 mSv, to prevent significant radiation exposure as part of standard radiation protection protocols.
Step-by-step explanation:
Protective aprons must be worn if one is likely to receive 5 millisieverts (mSv) or more per hour, as this is the typical threshold for when protective gear is required to prevent significant radiation exposure. This corresponds to option 2 which states 5 rad/hr, considering that 1 rad is equivalent to 0.01 J/kg and it closely approximates to 0.01 Sv or 10 mSv, meaning that 5 rad is therefore 50 mSv.
The usage of protective aprons is a crucial component of radiation protection measures, which also include minimizing time spent in radiation areas, maximizing distance from the source, and providing as much shielding as possible to lessen exposure.