A "figure eight" knot is primarily used to __________.

The figure eight knot is primarily used to keep a line from passing through a sheave. This type of knot creates a secure stop at the end of the rope, ensuring that it cannot slip through pulleys or blocks, which is essential in various boating situations where maintaining the integrity and position of the line is critical.

The design of the figure eight knot allows it to maintain a strong hold without slipping, making it an ideal choice for preventing lines from escaping through mechanical grips or sheaves. The knot's structure is also quite secure, which is crucial in scenarios where safety and reliability are paramount, such as in recreational and commercial boating operations.

While the options regarding being a stopper, shortening a line, and joining lines of equal size represent other useful knots and techniques, they do not accurately describe the primary purpose and effectiveness of the figure eight knot in maintaining control over a line’s movement through equipment.