Which locking nut is a type of free-spinning lock nut?

A Keps nut is a type of free-spinning lock nut that incorporates a built-in washer, allowing it to provide a locking mechanism without the need for additional components. The design typically features a serrated or toothed washer that grips the surface underneath it, helping to prevent loosening due to vibration. This unique construction makes it effective in applications where vibration may be an issue.

On the other hand, each of the other choices serves different purposes and does not fit within the category of free-spinning lock nuts. A flange nut, for example, has a wide flange that acts as an integrated washer, but it does not spin freely in the same locknut context. A castle nut includes notches for a cotter pin, which secures it but prevents it from being classified as free-spinning. Lastly, an acorn nut has a domed top for aesthetic purposes and protection but lacks mechanisms necessary for locking in the way a Keps nut does. Therefore, a Keps nut stands out as the correct answer.