A tank carrier designed to carry flammable liquids, combustible liquids, Class B poisons, and liquid food products with vapor pressures up to 3 psi, is an:

• A. MC 331
• B. MC 306/DOT 406
• C. MC 312/DOT 412
• D. MC 338

Answer: (B)

Vapor pressure dictates whether a tank needs to be pressurized or can stay non-pressurized. Liquids with a vapor pressure up to about 3 psi can be carried in a non-pressurized cargo tank. The MC 306/DOT 406 design is built for exactly that scenario: non-pressurized transport of flammable liquids, combustible liquids, Class B poisons, and certain liquid foods. The other tank types are designed for higher pressure or specialized services: low‑pressure tanks (MC 331) used when some forced pressure is acceptable, pressure tanks (MC 312/DOT 412) for corrosives, and cryogenic tanks (MC 338) for very cold liquids. So the described cargo matches the non-pressurized MC 306/DOT 406 designation.