How many license plate lamps and what color must they be on a trailer?

A trailer is required to have one license plate lamp that is either white or clear. This requirement serves the purpose of illuminating the rear license plate so it is visible in low-light conditions, ensuring compliance with traffic regulations. The color white or clear is standard as it provides optimal visibility without creating confusion for other drivers. These lamps are designed to clearly illuminate the license plate without emitting colored light that could be misinterpreted on the road.

Other options suggest either a red lamp, which is typically associated with rear lights or indicators, or yellow, which is not standard for license plate illumination. In the case of multiple lamps, state regulations specify that there is only a requirement for one lamp, further supporting the correctness of the chosen answer.