This knot is used to prevent the end slipping through a fastening or loop in another line. What is it called?

The correct choice is the figure eight knot. The figure eight knot is well-known for its ability to create a secure, non-slip loop at the end of a line, making it particularly useful in various rigging and lifting situations. Its structure consists of two loops that create a secure knot when pulled under tension, effectively preventing the end of the line from slipping through a fastening or loop in another line.

This makes the figure eight knot exceptionally reliable, especially in scenarios where maintaining a secure connection is critical, such as in climbing, sailing, or rigging applications. Its ease of control and the knot’s inherent security are why it is often highlighted for these purposes.

In contrast, the square or reef knot is primarily used for tying two ends of a rope together, which does not provide the same security for a single line passing through a loop. The bowline creates a fixed loop at the end of a line but is not used to prevent slipping through a fastening as effectively as the figure eight. The overhand knot, while simple and easy to tie, does not provide the same level of security or reliability for this specific application.