What is a key preflight check for the MCD on all CRJ aircraft?

A key preflight check for the MCD (Main Cargo Door) on all CRJ aircraft involves ensuring that the pins are not protruding. This step is crucial as it helps to confirm that the MCD is properly closed and secured before takeoff. If the pins were to be protruding, it could indicate that the door is not locked in place, which poses a safety risk during flight. Properly locking the MCD is essential for maintaining the aircraft's pressurization and safety protocols, making this check a vital part of preflight procedures.

While the other options mention important safety checks, they pertain to different aspects of the aircraft or cabin safety rather than specifically addressing the integrity and security of the MCD. Therefore, ensuring that the pins are retracted is particularly critical as it directly relates to the proper functioning of the cargo door.