When must the stability of a rocket be determined?

Determining the stability of a rocket when it is prepared for flight is crucial because this is the point at which all components are brought together and the rocket is subject to real-world conditions. Stability assessments at this stage ensure that the rocket will behave as expected during the flight, taking into account factors like weight distribution, aerodynamics, and center of pressure versus center of gravity. This final preparation stage is when the rocket's configuration is finalized, and any last adjustments can be made, allowing for a comprehensive assessment of stability prior to launch.

Understanding stability earlier, such as at the design stage or during assembly, is important but does not account for the complete integration of all components and the actual conditions the rocket will face. After the launch, while you can assess performance and stability through results, it is too late to make corrections or adjustments for that specific flight.

Thus, the correct choice highlights the importance of verifying stability in the actual flight-ready configuration, which is critical for a successful launch.