Which type of hydraulic pump uses a crescent design?

The internal gear pump utilizes a crescent design as part of its mechanism to move hydraulic fluid. In this type of pump, the design features an internal gear that is located eccentrically within the outer casing. The crescent-shaped piece fills the space between the gears, allowing the pump to achieve efficient fluid transfer.

This crescent shape is essential as it helps maintain a constant volume of fluid displacement during the rotation, ensuring a more consistent flow and pressure. Additionally, the design minimizes the risk of cavitation, as the crescent shape helps in trapping the hydraulic fluid effectively.

The other options, while also types of hydraulic pumps, do not utilize a crescent design. Gear pumps use interlocking gears without a crescent; vane pumps have vanes that slide out from a rotor, and piston pumps have cylinders and pistons that also do not feature a crescent geometry. The internal gear pump stands out in its reliance on this unique design for optimal function, which is why it is the correct answer to the question.