Which of the following is a common cause of flight delays?

Technical issues are a common cause of flight delays because they can significantly impact an aircraft's operational readiness and safety. These issues might include equipment malfunctions, mechanical failures, or other maintenance-related problems that require immediate attention before a flight can safely take off. When such technical problems arise, airlines must prioritize safety, which can necessitate delays while repairs are made or alternative arrangements are put in place.

The other options, while they can contribute to delays, are less frequent and often not as directly responsible for the immediate postponement of flights. Passenger preferences, for instance, may influence boarding times or seat assignments but are generally not a decisive factor in causing delays. Pilot availability could impact operations, but this scenario is typically managed through scheduling practices. Flight crew training schedules are necessary for safety and compliance but affect fewer flights in day-to-day operations. Thus, technical issues stand out as the most direct and common cause of delays in aviation.