What parameter must be checked for an OK takeoff according to the criteria?

For an acceptable takeoff, it is critical that the flaps are set to the correct position as indicated by the Multi-Function Control Display Unit (MCDU). This ensures that the aircraft has the appropriate aerodynamic configuration for takeoff, which is essential for achieving the necessary lift and performance characteristics at the moment of takeoff. Flaps help improve the lift-to-drag ratio and are vital for safe operations during this critical phase of flight.

If the flaps do not match the settings on the MCDU, it may indicate a pre-flight configuration error, which could have serious implications for the aircraft's performance and safety during takeoff. Confirming that flaps are correctly set as per the MCDU allows the pilot to ensure the aircraft is in optimal condition for departure.

Taking into consideration the other options: trim settings are usually monitored but should not be in the red range, indicating some adjustment is required; spoilers should be in the retracted position during takeoff; and the parking brake should be disengaged to allow for movement on the runway. Thus, verifying flap settings is essential for a safe and effective takeoff.