A miscellaneous opening that penetrates a perimeter must be protected and able to be locked...

Security standards typically require that any opening in a perimeter (e.g., HVAC ducts, pipe chases, vents) be lockable or otherwise protected if the cross-sectional area is 96 square inches or greater. This threshold is based on a size that could permit unauthorized entry or object passage.

A, B, and D are incorrect measurements based on security industry benchmarks.