Which of the following statements about monkey lines is true?

The statement that monkey lines are essential for crew safety during hoisting operations is accurate because monkey lines are specifically designed to provide additional safety and support to crew members during such operations. They are utilized as part of a safety system to prevent falling or to assist crew members in maintaining their position while on board or during critical tasks.

In the context of hoisting, monkey lines provide a clear point of attachment that enhances stability and control while managing the risks associated with working at height or near the edge of a vessel. Their purpose is not only functional in maintaining crew members' safety but also crucial in ensuring they can execute their tasks effectively without compromising their well-being.

The other options miss essential aspects of monkey lines: they are not solely reliant on calm waters since they are designed for various conditions, they are certainly not decorative but rather functional, and while they can provide support, they are not designed to hold a complete load equivalent to 100% of a person’s weight; they are best used in conjunction with other safety equipment.