When must you dim your high-beam lights when approaching another vehicle?

Answer: C — 500 feet of an oncoming vehicle

Dimming your high-beam lights when approaching another vehicle is required at a distance of 500 feet. This regulation exists to prevent glare that can impair the vision of other drivers, making the roads safer for everyone. High-beam lights can be much more intense than low-beams, and shining them directly at an oncoming driver can momentarily blind them, increasing the risk of accidents. By dimming your lights at this specific distance, you give the oncoming driver time to adjust to your vehicle's approach without being temporarily blinded. Therefore, it is crucial to follow this guideline to promote safety on the road. The distances provided in the other choices do not provide enough margin to ensure that oncoming drivers are not affected by high-beam glare, hence they are not the correct choice.