Final answer:
The 1-mile run test is the most commonly used assessment of aerobic fitness for a 14-year-old boy, being simple to administer and providing a clear measure of cardiovascular endurance.
Step-by-step explanation:
The assessment of aerobic fitness that is most commonly used for a 14-year-old boy is the 1-mile run test. This test is preferred for this age group because it is straightforward to administer, can be easily performed on a track, and provides a clear measure of cardiovascular endurance based on the time it takes to complete the distance. Other tests, such as the step test and the 12-minute walk/run, are also used but may not be as common for this specific age group as the Cooper test, which is more geared towards adults and highly fit individuals.