What action must be taken if the landing gear doors are not closed before takeoff?

If the landing gear doors are not closed before takeoff, the appropriate action is to abort the takeoff. This is crucial because open landing gear doors can lead to increased drag and could result in aerodynamic issues, compromising the aircraft's performance during the critical phase of takeoff. Ensuring that all systems are functioning correctly and that the aircraft is configured properly is essential for safe flight operations.

Other actions, such as initiating a go-around or performing a visual inspection, would not be applicable as these do not address the immediate risk associated with an improper configuration for takeoff. Continuing the climb would put the aircraft and crew at significant risk, as the aircraft would not be operating under optimal conditions. Therefore, aborting the takeoff is the necessary step to ensure safety before proceeding.