A round, yellow sign with a large "X" and the letters 'RR' accompanied by a smaller rectangular sign that reads "EXEMPT" means:

Answer: B — stop is not required except when a train is approaching the crossing

A round, yellow sign with a large "X" and the letters 'RR' signifies a railroad warning, indicating that drivers should be aware of railway crossings. The additional rectangular sign that reads "EXEMPT" indicates that specific vehicles are not required to stop at this railroad crossing. This typically applies to certain types of vehicles, such as buses or trucks transporting certain materials, which may have different stopping requirements at railroad crossings compared to standard vehicles. This means that while there is a general warning of a railroad crossing, the "EXEMPT" sign clarifies that for the vehicles specified, a stop is not mandated, except when there is an actual train approaching. Understanding this detail is crucial for drivers to ensure they navigate the crossing safely without unnecessary stops while still complying with the law. In contrast, if the sign indicated a requirement for a stop or signalized that trains have a right of way, different actions would be necessary. This highlights the importance of recognizing specific roadway signs and what they imply for vehicle operation.