Hazardous materials (dangerous goods) are categorized into nine distinct classes based on their primary physical, chemical, and toxicological properties. This classification system is established by the United Nations Recommendations on the Transport of Dangerous Goods and is adopted by national regulatory bodies, such as the U.S. Department of Transportation (DOT). NFPA standards, including NFPA 400, Hazardous Materials Code, reference this 9-class system for consistent hazard identification and management in storage, handling, and use, ensuring a standardized approach to safety and emergency response.

A. 5: Incorrect. This number does not align with the internationally recognized UN/DOT system for classifying hazardous materials, which has nine primary classes.

B. 7: Incorrect. This number is not the standard for hazardous material classes. The established system is based on nine distinct hazard categories.

D. 11: Incorrect. While some of the nine classes are further broken down into divisions, the total number of primary, numbered classes is nine.

1. NFPA 400, Hazardous Materials Code, 2022 Edition. Section 4.1.1, "General," states that hazardous materials shall be classified in accordance with the U.S. Department of Transportation (DOT) regulations, which utilize the nine-class system.

2. U.S. Department of Transportation, Code of Federal Regulations, Title 49 (49 CFR) § 172.101, "Purpose and use of hazardous materials table." The table and its preceding sections define the nine hazard classes (e.g., Class 1-Explosives, Class 2-Gases, etc.).

3. SFPE Handbook of Fire Protection Engineering, 5th Edition. Chapter 59, "Hazardous Materials," Section 59.2.1, "Classification," explicitly lists and describes the nine UN/DOT hazard classes. (DOI: 10.1007/978-1-4939-2562-459).