What was the reason for the delays in all flights?

The correct answer points to a strike by check-in staff as the reason for the delays in all flights. Strikes can significantly impact airline operations, especially if they involve essential staff members like check-in personnel who handle passenger services, ticketing, and boarding processes. When these staff members are on strike, it can lead to a backlog of customers unable to check in or board flights, resulting in widespread delays across multiple flights.

This context illustrates why the other options do not capture the primary reason for the delays. Weather conditions can indeed cause flight interruptions; however, the question specifically asks for the reason behind the delays, suggesting a more direct cause rather than a seasonal or atmospheric factor. Technical issues with aircraft are typically isolated incidents affecting individual flights rather than causing system-wide delays. Lastly, while safety checks being conducted can slow down operations, they are part of standard procedures and unlikely to halt flights entirely in the event of a staff strike. Overall, selecting the correct answer reflects an understanding of how labor disputes significantly disrupt airline schedules.