The Yellow Section of the ERG should be used when:

The Yellow Section of the Emergency Response Guidebook (ERG) is specifically designed to provide information based on the United Nations (UN) numbers associated with hazardous materials. When responders know the UN number, they can quickly look up the relevant guide in the Yellow Section of the ERG to find critical information about the chemical, including its hazards, emergency response information, and safety measures that need to be taken.

Understanding this aspect of the ERG is crucial for effective and timely response to hazardous materials incidents. The other choices involve scenarios that do not align with the specific purpose of the Yellow Section. For instance, if the chemical identification number is unknown, the Yellow Section cannot be used effectively because it relies on the known UN number. Similarly, while public safety concerns and emergency response information are important in an emergency response context, they are broader elements addressed in various sections of the ERG, not limited specifically to the Yellow Section.