Which preflight action is required for every flight?

The requirement for preflight action is rooted in ensuring safety and compliance with aviation regulations. The correct answer involves determining the runway length at the airports of intended use, which is crucial for a safe flight operation. Knowing the runway length is necessary for evaluating whether the aircraft can take off and land safely, particularly when considering factors such as aircraft weight, weather conditions, and altitude.

This information helps pilots assess if the chosen runway is adequate for the aircraft's performance capabilities. For instance, if the runway is too short for takeoff or landing, it could pose a significant risk, leading to a potential accident.

In contrast, while checking weather reports, determining alternatives, and checking flight documents are all important aspects of flight preparation, they are not universally mandated for every flight in the same way that runway length must be considered. Weather conditions can vary, alternatives can be assessed based on specific circumstances, and flight documents may already be in order. However, runway length is a fundamental requirement that directly impacts the safety and feasibility of every flight, making it essential for all pilots to ascertain before taking off.