Which knot is classified as a stopper knot?

The overhand knot is classified as a stopper knot because its primary function is to prevent a rope from slipping through a device or a loop. When tied, it creates a bulky knot at the end of the rope, effectively stopping any further movement in that direction. This characteristic is crucial in various applications, such as securing loads or ensuring that a line doesn't pull through a pulley or other hardware.

In the context of rope rescue and other climbing-related activities, knowing how to effectively utilize a stopper knot like the overhand knot is essential for safety and efficiency. It's a simple yet highly effective knot that even beginners can learn to tie quickly, making it a staple in any ropework scenario.

Other knots, like the butterfly knot or figure 8 knot, serve different purposes and are not primarily intended as stopper knots. The double fisherman's knot is mainly used for joining two lines together rather than acting as a stopper. Therefore, the overhand knot is the best answer when identifying a stopper knot.