What device is used to close off the opening of a hook?

The device used to close off the opening of a hook is the latch. Latches are integral safety features on hooks that prevent the load from accidentally slipping off during lifting operations. When a load is attached to the hook, the latch secures the load in place, ensuring that it cannot fall or become disengaged from the hook while being hoisted. This is essential for maintaining safety and preventing accidents in rigging scenarios.

In contrast, the other options serve different purposes: a swivel allows for rotational movement in the rigging system, a shackle is used for connecting different components or tying off a load, and a nut typically serves fastening purposes rather than closing off a hook. Understanding these distinctions is essential for effective and safe rigging practices.