Which of the following is used to prevent a line from running through a block?

The correct choice is the Figure Eight knot, which is commonly used in various nautical and climbing applications to prevent a line from running through a block or otherwise coming undone. When tied correctly, the Figure Eight creates a secure stop at the end of the rope, which means it effectively prevents the line from slipping or passing through the block.

This knot is favored for its reliability and ease of untie after being under load, making it practical for situations where secure fastening is needed yet the ability to release the knot easily is also important. The structure of the Figure Eight provides a substantial amount of friction against the block, securing the line in place.

In contrast, other types of knots like the Square Knot or Sheet Bend serve different purposes, such as joining two ropes of similar or different diameters rather than acting as a stop. Similarly, while a Round Turn with Two Half Hitches is useful for securing a line around an object, it does not provide the same stop mechanism at the end of a line as the Figure Eight does. Therefore, the characteristics of the Figure Eight make it the most suitable choice for preventing a line from running through a block.