Which letter indicates that a material is regulated only by domestic transport?

The correct answer indicates that the material is regulated only by domestic transport. This is significant because materials that are categorized this way are controlled by national regulations rather than international standards. This often applies to substances that pose a potential risk within a specific country but may not be subject to the same level of scrutiny or regulation when it comes to international transport.

In the context of hazardous materials, this means that the substance can be safely transported within the nation but does not meet the criteria for more stringent regulations applicable to international shipping. Understanding this distinction is crucial for compliance and safety when dealing with hazardous materials that may vary in classification based on transport routes.

The other options indicate different regulations or are affiliated with other transport requirements, focusing on either international standards or specific classifications that imply broader regulatory oversight.