What types of materials are regulated under the Hazardous Materials Transportation Act (HMTA)?

The correct answer focuses on substances defined as hazardous according to specific regulations. The Hazardous Materials Transportation Act (HMTA) is designed to ensure safe and secure transportation of hazardous materials, which can include a wide range of substances that pose risks to health, property, or the environment during transit. These substances are categorized based on their potential hazards, such as flammability, toxicity, and reactivity.

This approach enables regulatory bodies to establish safety protocols, labeling requirements, and training for handlers and carriers of these materials. The emphasis on "any substance defined as hazardous by regulations" reflects the comprehensive nature of the HMTA, which encompasses various materials that meet specific criteria set forth in federal regulations.

In contrast, focusing solely on biologically hazardous materials, all materials regardless of risk level, or only those transported internationally does not align with the intent and scope of the HMTA. The act specifically targets hazardous materials, which can include but are not limited to biological agents. Therefore, the answer captures the full breadth of hazardous materials indicated by the HMTA, making it the most accurate choice.