Which of the following is a requirement for all drivers on public roads?

Having a valid driver's license is a fundamental requirement for all drivers on public roads. This ensures that drivers have demonstrated the necessary knowledge of traffic laws, road signs, and safe driving practices. A driver's license serves as proof that the individual has met the qualifications to operate a vehicle legally.

While being 21 years old or older may apply to specific circumstances, like certain rental car agreements, it is not a universal requirement for all drivers. Similarly, while car insurance is mandatory in many states, it is not a requirement for all drivers in some jurisdictions. Lastly, passing a driving test every year is not standard practice; typically, drivers are required to pass the test only when obtaining their license or when renewing it after a certain period. Thus, maintaining a valid driver's license is the essential prerequisite for legal driving on public roads.