What is used to hold the landing gear up?

The correct choice is hydraulic pressure. In multi-engine aircraft, the landing gear system typically relies on hydraulic power to function. Hydraulic systems are designed to handle large forces with relatively small components, making them efficient for operating various mechanisms, including the raising and lowering of landing gear. When the landing gear is retracted, hydraulic pressure is applied to retract the gear into the aircraft's fuselage, allowing for a streamlined aerodynamic profile during flight.

Once the landing gear is fully retracted, mechanical locks may engage to keep the gear in that position, but the primary force keeping the landing gear up is the hydraulic pressure. This hydraulic system allows for smooth and reliable operation, reducing pilot workload compared to manually operated systems.

While electrical power might be used to activate hydraulic pumps or control systems, it is not directly responsible for holding the landing gear up. Mechanical locks, while important for safety, do not provide the initial force to lift the gear; they simply secure it in place once retracted. Gravity also does not hold the landing gear up; rather, it acts in the opposite direction, which is why hydraulic pressure is essential to overcome that force when the gear is uplifted.