Which of the following hazardous materials must a bus NEVER carry, in any quantity?

AAMVA CDL Manual Section 4.2.2 "Forbidden Hazardous Materials," p. 4-2

Not every hazardous material has a weight exception; some are banned outright because even a small leak or breach in an occupied vehicle is too dangerous. Knowing which materials fall into this absolute-ban category, versus which just have a quantity limit, is a distinction the exam draws directly.