A disadvantage of a ratchet jack is:

A ratchet jack is designed to provide mechanical lifting capabilities, often utilized in vehicle rescue scenarios. One of the key disadvantages of using a ratchet jack lies in its stability under load. When a ratchet jack is raised, it relies on a series of notches and a pawl to hold the load. This mechanism can become unstable, especially if the load shifts or there is any lateral movement, which can lead to a risk of tipping or failure. Ensuring stability while lifting is critical in rescue situations, as any instability can compromise safety for both the rescuer and the victim.

In contrast, while a limited lift span does restrict the height that can be achieved, it does not necessarily affect the overall stability. The fact that a ratchet jack does not retract on its own is a characteristic of its design rather than a disadvantage in terms of its functional stability. Finally, identifying it as a mechanical device does not inherently imply a lack of safety or reliability; many mechanical devices are designed with stability in mind. Overall, the least stable under load characteristic makes option C the most relevant disadvantage of a ratchet jack in rescue operations.