Which of the following is the electric device of the stacker?

The ignition device is the correct answer because, in the context of stacker equipment, it refers to the electrical components that are responsible for starting and managing the energy system of the stacker. Although stackers, especially electric-powered ones, do rely on electrical systems for operation, the terminology focuses specifically on how electric power is initiated and utilized within the equipment.

When discussing the other options, the brake lever is a mechanical component responsible for stopping the stacker and does not relate to the electrical systems. Wheels are crucial for mobility but are not tied to electric functionality. The differential is typically found in machines with variable speed control and does not pertain specifically to the electrical device aspect of a stacker. Thus, the ignition device stands out as the key electrical element of the system.