Final answer:
Blood-soaked gauze pads should not be placed in a plastic biohazard bag due to their need for specialized disposal such as incineration or autoclaving, in contrast to other contaminated items like IV catheters or airways which can be disposed of in biohazard bags.
Step-by-step explanation:
When cleaning the back of a truck after transporting a trauma patient, it is crucial to handle contaminated items with care to prevent the spread of infection. Among the items listed, blood-soaked gauze pads should not be placed in a plastic biohazard bag but instead should be handled as regulated medical waste and require special disposal methods such as incineration or autoclaving. Items like a plastic IV catheter, nasopharyngeal or oral airway, and French suction catheter are typically disposed of in the plastic biohazard bags as they are considered contaminated but not as heavily soaked with blood that they would need more rigorous disposal methods.