In which type of conveyance system is energy applied by means of a liquid under pressure?

The correct answer is hydraulic elevator. A hydraulic elevator operates by using a fluid, typically oil, that is pumped into a cylinder beneath a piston. The pressure from the fluid causes the piston to rise, thereby lifting the elevator car. This system is particularly suitable for low-rise buildings due to its ability to efficiently lift heavy loads. Hydraulic elevators are characterized by their smooth and quiet operation, which is a result of the controlled movement of the hydraulic fluid.

In contrast, a pneumatic elevator uses air pressure to lift the cab, which operates differently from a hydraulic system. A traction elevator relies on cables and pulleys, with mechanical energy applied to move the elevator, rather than using pressurized liquid. The vacuum elevator utilizes a difference in air pressure to create lift, which again diverges from the principle of applying liquid under pressure. This distinction highlights why the hydraulic elevator is the correct choice for a conveyance system where energy is applied specifically by means of a liquid under pressure.