Which knot is often used as a stopper knot?

The figure 8 knot is commonly used as a stopper knot due to its structure and reliability. This knot is formed by creating a loop and passing the working end of the rope through it, which results in a strong and secure configuration. The figure 8 knot is effective in preventing ropes from slipping through anchor points or other knots, which is essential in firefighting and rescue operations to ensure safety.

The design of the figure 8 knot not only provides a secure hold but is also easy to untie after being under load. This characteristic is particularly important in emergency situations where time and efficiency are critical. Furthermore, the knot's bulkiness makes it easily recognizable and accessible, which contributes to its effectiveness as a stopper knot.

In contrast, the other knots mentioned serve different primary purposes. The clove hitch is primarily used for securing a rope to an object, while the square knot is designed for tying two ropes together. The bowline knot, known for creating a fixed loop at the end of a rope, is also not primarily intended to function as a stopper. Thus, the figure 8 knot stands out as the most suitable choice for this specific application.