Which of the following safety items must be used when the cab windows of a backhoe are open?

In situations where the cab windows of a backhoe are open, the most critical safety item to use is a hard hat. This is because hard hats provide essential protection for the head against falling objects or debris, which can be more likely when windows are open. Additionally, working on a job site often involves potential hazards that could lead to head injuries, so wearing a hard hat is a standard safety practice in construction and heavy equipment operation.

While ear protection, reflective jackets, and tool belts are all important safety items, they do not address the specific risk associated with open cab windows. Ear protection is important in noisy environments but is less relevant when considering open windows. Reflective jackets enhance visibility on job sites but do not provide head protection. A tool belt is necessary for carrying tools but does not offer personal safety against the dangers posed by an open cab. Thus, the hard hat is the most appropriate safety item in this context.