Which type of elevator system relies on hydraulic fluid to lift the cab?

The correct choice identifies the hydraulic elevator system, which operates based on the principle of hydraulic fluid to lift the cab. In this system, a pump forces hydraulic fluid into a cylinder, which moves a piston upward, thereby raising the elevator cab. This method allows for the lifting of heavy loads with relatively less energy compared to other systems.

Hydraulic elevators are particularly advantageous for low- to mid-rise buildings, as they provide a smooth ride and can handle heavier capacities without the complexities involved in traction systems. The reliance on hydraulic fluid distinguishes these elevators from others.

In contrast, traction elevators utilize cables and pulleys to elevate the cab and do not employ hydraulic fluid. Pneumatic elevators rely on air pressure and vacuum to function, and rope elevators involve various types of mechanisms that generally do not involve hydraulic systems for operation. Understanding the mechanics behind each type of elevator enhances comprehension of their individual applications and functionalities in elevator design.