Which of the following hazard classes should NOT be placed in a temperature-controlled trailer?

The correct choice indicates that certain hazard classes should not be transported in a temperature-controlled trailer. Classes 1 (Explosives), 2.1 (Flammable gases), and 3 (Flammable liquids) represent materials that have specific safety requirements and risks associated with them.

Transporting explosive materials (Class 1) requires strict adherence to safety protocols to prevent detonation risks, which can be exacerbated by improper temperature control or handling. Class 2.1, which includes gases that can ignite easily, also poses a significant risk during transit, as their flammability could be impacted by incorrect temperature ranges. Meanwhile, Class 3 materials are flammable liquids that can become volatile under certain conditions, potentially causing spills or fires.

For these hazard classes, using a temperature-controlled trailer could create an unnecessary risk; therefore, they must be managed under strict regulations that often do not include temperature adjustment. Other classes listed in the other answer options do not carry the same level of risk that would render them unsuitable for temperature-controlled transport. For example, Classes 4 (Flammable solids) and 5 (Oxidizers) have different transportation requirements that may not necessitate temperature regulation in the same critical way.