Which of the following substances CANNOT be shipped via United States Postal Service (USPS)?

• A. Category B substances
• B. Nonhazardous materials
• C. Category A substances
• D. Water-based liquids

Answer: (C)

The main idea is understanding USPS limits for mailing dangerous goods, especially infectious substances. Category A infectious substances are those that can cause severe or fatal disease in healthy people or animals and require the most stringent biosafety and transport controls. They are regulated under strict packaging and transport rules (IATA/ICAO, triple packaging, trained handlers, and transport by approved carriers), and they are not allowed to be shipped through the United States Postal Service. That makes them unshippable by USPS.

In contrast, the other options are either nonhazardous or lower-risk infectious substances that can be mailed under specific USPS guidelines if properly packaged and declared. Nonhazardous materials are routinely mailed, and water-based liquids can be mailed when they don’t present a hazard and are packaged according to the applicable rules. But because Category A substances require a level of transport safety and access that USPS does not provide, they cannot be shipped via USPS.