A motor vehicle's headlamps must be mounted at a height between __________ inches and __________ inches from the ground.

The correct answer indicates that a motor vehicle's headlamps must be mounted at a height between 24 inches and 54 inches from the ground. This regulation ensures that the headlamps are positioned adequately for maximum visibility while also preventing glaring light that could distract or impair the vision of oncoming drivers.

Mounting headlamps within this specified range strives to strike a balance between effective illumination of the roadway and compliance with safety standards designed to minimize hazards caused by improperly positioned lighting. Having the headlamps set too low might lead to inadequate visibility, especially in adverse weather conditions, while positioning them too high could result in harsh glare for other drivers.

Other height ranges provided in the options are either too low or too high, which would not be compliant with the recognized safety standards set for motor vehicle lighting. Each of those ranges fails to meet the criteria ensuring that vehicles can operate safely and effectively on various roadways.