Except for the driver of the stacker, which of the following statements is correct?

The statement that a stacker cannot embark personnel is correct because forklift trucks, including stackers, are specifically designed for material handling and not for transporting people. Safety regulations typically prohibit carrying passengers on such vehicles due to the risks associated with instability and the potential for accidents. Load capacity and distribution are carefully calculated for cargo, not for accommodating personnel, which could lead to unsafe driving conditions and increased chances of tipping over.

In contrast, the other options suggest scenarios that either do not align with the intended use of stackers or could lead to unsafe practices. For example, allowing passengers or multiple loads could compromise the stability and control of the vehicle. Operating in reverse is a common function for stackers, but it does not directly address the safety regulation about personnel transport, hence does not contradict the premise of the question about who can be on the stacker. Thus, the correct choice emphasizes the safety-focused design and operational guidelines for stackers, highlighting their primary function in material handling over personnel transport.