What items must be present before a barrel of hazardous chemicals can be shipped?

The correct answer is based on the requirement that before shipping hazardous chemicals, specific documentation and labeling must be in place to ensure safety and compliance with regulations. The Safety Data Sheet (SDS) serves as a crucial resource detailing the hazards associated with the chemical, including necessary handling and emergency measures. Additionally, a warning label is vital as it communicates important information about the risks and precautions that handlers must take when dealing with the hazardous material.

The presence of an SDS is particularly important because it must accompany shipments of hazardous substances, providing handlers with critical information about the chemical's physical and health hazards, as well as guidance on safe practices. Without these two elements—a comprehensive SDS and an appropriate warning label—the risks posed by hazardous materials could lead to unsafe handling, which is why they are mandated by OSHA regulations and other safety standards.

The other options, while relating to safety in some manner, do not cover the essential requirements adequately as mandated by safety regulations for shipping hazardous materials.