When must the headlight be displayed while in yard service?

The correct choice indicates that the headlight must be displayed to the front and rear at all times while in yard service. This requirement ensures that the train is visible to other trains and workers in the yard, enhancing safety overall.

Displaying headlights at all times in a yard setting is crucial because trains may be moving in various directions, and yard environments can be busy places with other rail vehicles, maintenance crews, and safety personnel present. By keeping the headlight on for both the front and rear, it minimizes the risk of accidents by making the train's position known to others.

Other choices suggest limited use of headlights, which would undermine safety in the yard. Using headlights only at specific times, when in motion, or while approaching stations could lead to situations where visibility is compromised, potentially resulting in dangerous encounters or accidents.