Which of the following is a condition for using airways?

The correct condition for using airways is having the correct navigation equipment. Airways are structured routes in the sky that require pilots to navigate precisely using specific ground-based navigation aids or GPS. It is essential for pilots to be equipped with appropriate and functioning navigation systems to ensure safe and accurate adherence to these routes, as well as to maintain contact with air traffic control and to successfully follow any assigned air traffic procedures.

While the other options present good practices for flying in general, they do not specifically pertain to the requirements for flying on established airways. Having a valid student pilot certificate, completing a pre-flight inspection, or filing a flight plan are important for general flight operations but do not directly address the specific need for navigation capabilities that are critical for safe navigation within airways.