Which item is used to prevent two moving parts from separating?

The correct answer is Keeper. A keeper is designed specifically to prevent two moving parts from separating, typically in mechanical systems. It acts as a securing device that holds components in place, ensuring they remain engaged during operation. This function is crucial in various applications where the integrity of the assembly is essential for safe and proper functioning.

For example, in machinery and automotive applications, a keeper can be used to retain shafts, gears, or other rotating elements, thus preventing them from dislocating under operational stresses. The term "keeper" is often associated with retaining devices that are critical in maintaining alignment and preventing wear due to misalignment or detachment.

In comparison, while items like cotter pins, hooks, and various types of pins can also serve to secure components, they typically fit within a broader context of fastening or connecting but may not specifically focus on preventing separation in moving parts to the same extent as a keeper. Therefore, while they play their own roles in assembly and stability, the keeper is the most precise tool for this particular function.