For nighttime operations, traffic cones should have a minimum height of _____ inches and must be _____.

The correct choice highlights that for nighttime operations, traffic cones should have a minimum height of 28 inches and must be retroflective. This is essential for ensuring visibility in low-light conditions, allowing drivers to see the cones and the traffic control setup from a distance, which enhances safety in the work zone. Retroreflectivity allows the cones to reflect light from headlights back towards the vehicle, improving their visibility significantly after dark when compared to non-retroreflective materials.

Using cones of adequate height ensures they are also noticeable to drivers in various vehicles, including larger ones like trucks or buses. This combination of appropriate height and retroreflective material is crucial for effective nighttime traffic management, which ultimately aids in reducing the likelihood of accidents in work zones during these hours.