What is the only exception to the general statement that BUR systems are not suitable to leave unprotected?

The correct choice indicates that an asphalt BUR with a thin coating of asphalt, referred to as a glaze coat, is the exception to the norm that BUR systems typically require protection. BUR systems generally need a protective layer, such as gravel or other materials, to safeguard them from environmental factors like UV radiation and physical damage.

The glaze coat serves as a protective layer while maintaining the benefits of the BUR system. This thin coating can offer some level of resistance to weathering and can enhance the UV protection of the underlying asphalt layers. As a result, it can occasionally be left unprotected compared to other BUR configurations that necessitate additional surfacing for durability and longevity.

For the other options, a flood coat of bitumen with a gravel surface typically requires protection from environmental elements to ensure its effectiveness and lifespan. An adhered aggregate without bitumen wouldn’t meet the necessary standards for protection, nor would a standard underlayment without additional coating be adequate. The distinction made for the asphalt BUR with a glaze coat showcases its unique capability to withstand exposure without further surfacing compared to other methods that inherently rely on added protective measures.