What indicates that cargo contains hazardous materials?

The correct answer is a four-inch, diamond-shaped hazardous materials label. This label is a standardized system used to identify hazardous materials being transported. The specific colors and markings on the diamond shape indicate the type of hazard, such as flammable, toxic, explosive, or corrosive materials. This labeling is essential for the safety of drivers, emergency responders, and the general public, as it clearly communicates the potential risks associated with the cargo.

Other options, such as a small sticker on the door or a general cargo inspection tag, do not specifically indicate hazardous materials. These may provide various forms of information about the vehicle or cargo but lack the standardized identification that the diamond-shaped label provides. A unique vehicle identification number is related to the vehicle itself and does not pertain to the nature of the cargo being carried. Thus, the diamond-shaped hazardous materials label is the appropriate and recognized way to signify hazardous cargo.