What device is used to attach the chain so that the anchor line can spin freely?

The correct choice is the swivel, which is a crucial component in anchoring systems. A swivel is designed to allow the anchor line to spin freely without twisting or tangling, maintaining the integrity and functionality of the entire anchoring system. This is particularly important in situations where the vessel may experience movement due to wind or water currents; a swivel prevents the anchor line from becoming twisted, which could lead to difficulties in retrieving the anchor or, in worse cases, loss of the anchor altogether.

By using a swivel, boat crews ensure that the chain or rope attached to the anchor can rotate without restriction, allowing for smoother operation and easier handling. This device also reduces wear and tear on the anchor line, which can be susceptible to chafing and twisting in the absence of a rotating mechanism.

In contrast, a shackle, clip, or pin serve different functions—such as connecting various components of the anchoring system but do not facilitate the rotational capability necessary for the anchor line to operate effectively when under strain. The specific design and purpose of a swivel in allowing movement while preventing tangling makes it the appropriate choice for this application.