What defines the start of holdover time during flight operations?

The correct choice highlights that holdover time begins at the start of the final de-ice or anti-ice fluid application. This is crucial because holdover time refers to the period during which de-icing or anti-icing fluids are effective in preventing ice accumulation on the aircraft's critical surfaces. Proper timing of the fluid application is critical to ensure safety and performance, as the effectiveness of these fluids diminishes over time, especially in adverse weather conditions.

Starting the holdover time at the point of fluid application allows for accurate monitoring and decision-making regarding the aircraft's readiness for takeoff. This helps ensure compliance with safety standards and provides pilots with the information they need to manage ice control and maintain aircraft performance during flight operations.