Which of the following would prevent a driver from being certified by a medical examiner?

The ability to distinguish among traffic devices showing red, amber, and green colors is crucial for safe driving and is a specific requirement within the vision standards set for commercial drivers. A driver's capacity to accurately identify and respond to these color signals is essential not only for their safety but also for the safety of others on the road. If a driver cannot differentiate between these important signals, they would pose a significant risk while operating a vehicle, which would prevent them from being certified by a medical examiner.

In contrast, the other options describe minimum visual acuity standards that are necessary for certification, but they do not immediately disqualify a driver from being certified if they are met. This means that while options A, B, and C focus on visual acuity and field of vision, they do not address a critical functional ability that directly impacts driving safety in the same way that the ability to discern traffic light colors does. Hence, the inability to distinguish between these colors represents a direct and significant impairment affecting a driver's fitness to operate a commercial vehicle.