A prosthesis is:

A prosthesis is defined as an artificial body part designed to replace a lost or absent limb or function, which makes the option describing it as such the correct response. Prostheses can serve various purposes, including aiding mobility or restoring function to an area of the body that has been amputated or lost.

The other options do not accurately represent what a prosthesis is. While a device used to support a part of the body could describe a prosthetic device in certain contexts, it does not capture the specific nature of a prosthesis, which is fundamentally concerned with replacement rather than mere support. Similarly, categorizing a prosthesis as an activity of daily living misses the mark, as it is not an action but rather a tool that aids in performing those activities. Lastly, while cushioning devices may help in providing comfort, they do not have the functional purpose of replacing body parts, which is the essence of what makes a prosthesis distinct.