Tankers that are used to transport hazardous materials on vehicles must be marked with the ____ number of the product.

The correct choice is to use the UN or NA ID number when marking tankers that transport hazardous materials. The UN (United Nations) number is a four-digit classification number assigned to hazardous substances and articles for transport. This system is designed to provide a standardized method for identifying dangerous goods during transportation, enabling emergency responders to quickly understand the risks associated with a particular substance.

Having the UN or NA ID number displayed on tankers ensures that anyone handling the transportation or involved in an emergency response has immediate access to critical information about the material being carried. This can significantly aid in both safety and procedural effectiveness during the handling of hazardous materials.

The other options, while relevant in different contexts, do not fulfill the specific requirement for marking hazardous materials in transportation. CAS numbers refer to the Chemical Abstracts Service and are primarily used in regulatory contexts but are not suitable for marking transport vehicles. HSN (Hazardous Substance Number) and PIN (Product Identification Number) are also not standard requirements for the transport markings as defined by transportation regulations. Thus, using the UN or NA ID number is essential for compliance and safety in the transportation of hazardous materials.