Propane is used in the CDL Manual as the example material for which hazard class and division?

AAMVA CDL Manual, Section 9.3.1, Figure 9.1 (nhcdm.pdf p. 9-2); 49 CFR 173.115

Recognizing common materials like propane by their correct class and division is exactly how the real test questions and real roadside placarding decisions work — you identify the material, then look up its class to know which placard and handling rules apply.