What should not block the smoke detector according to PDIs?

The correct answer highlights that nothing should obstruct the smoke detector. This principle is essential for ensuring that the smoke detection system operates effectively and can timely alert the crew to any fire or smoke hazards. Smoke detectors are vital safety devices on an aircraft, designed to identify smoke and potentially prevent disastrous situations. Any obstruction could impede their functionality, leading to delayed responses in emergencies.

While passengers' personal items, a safety cover, and outside smoke may pose various concerns, none of these should obstruct the smoke detector. It's critical in aviation safety protocols to maintain clear access to smoke detectors at all times, thus reinforcing the importance of option B in maintaining safety standards.