Which lifting device has the simplest construction and uses the basic lever-and-fulcrum principle?

The ratchet jack exemplifies the simplest construction among lifting devices and is fundamentally based on the lever-and-fulcrum principle. This device operates using a lever that pivots on a fulcrum, allowing users to lift heavy loads with minimal effort by providing mechanical advantage.

As the user turns the handle, the engagement of the ratchet mechanism facilitates movement in only one direction, allowing the load to rise steadily. This design relies primarily on the principles of levers, where a smaller force applied over a distance results in a larger force that lifts the weight.

The other options, like the mechanical hoist, chain block, and pneumatic jack, while effective in lifting, are built with more complex mechanisms that incorporate additional components such as gears or powered systems. These devices do not adhere as strictly to the basic lever-and-fulcrum concept in their operation. The ratchet jack's straightforward design and operation make it an excellent example of leveraging simple physical principles for lifting tasks.