Which of the following is a special knot on a hackamore?

The fiador is a specialized knot commonly associated with hackamores, especially in the context of its use in bridles. It serves an important function in providing stability and security to the hackamore setup. The fiador knot is generally tied around the horse's head, allowing the reins to pull in a way that controls the horse without putting pressure on its mouth, making it ideal for horses that cannot tolerate a bit. This knot is designed to be a secure point of attachment that makes it easier for the rider to manage the horse while maintaining a comfortable fit.

The other knots mentioned, such as the bowline, square, and clove hitch, serve different purposes in various applications of knot tying but are not specific to a hackamore. Each of those could be used in other contexts, such as securing items, making loops, or fastening, but they do not provide the unique advantages that a fiador offers in the realm of horse tack.