Which lubricating system relies entirely on gravity?

The gravity lubrication system is designed to rely solely on the force of gravity to feed lubricant to the components that require it. In this system, the lubricant is stored in a reservoir positioned above the parts needing lubrication, allowing it to flow downwards due to gravitational pull. This method is relatively simple and low-maintenance because it does not require pumps or complex mechanisms to operate.

In contrast, the forced lubrication system actively uses a pump to circulate oil throughout the lubrication circuit, ensuring that oil is delivered to various components under pressure. The splash lubrication system uses moving parts, such as rotating components, to splash lubricant around the engine's interior, which is reliant on the motion of these components rather than gravity. The drip lubrication system involves controlled dripping of oil onto parts from a higher point but operates under different principles than gravity alone, as it may also rely on additional mechanisms to ensure proper oil distribution.

Thus, the gravity lubrication system stands out as the only one that operates solely on the principle of gravity, making it the correct choice in this context.