What type of moor uses a large ring or collar that moves vertically with the tide?

The correct answer involves the type of mooring that utilizes a large ring or collar capable of moving vertically with the tide, which is characteristic of the spud. A spud is a vertical pole that is anchored to the seabed and adjustable, allowing it to accommodate changing water levels due to tides. This flexibility is crucial, especially in areas where tidal variations are significant, as it helps maintain stability and reduces the risk of the vessel becoming unmoored or misaligned.

In contrast, other mooring types mentioned do not have this vertical movement feature. A fluke moor typically involves a more permanent anchoring system designed for stability without adjusting to tide changes. A bitt moor is used for securing lines or cables but does not involve the vertical movement of a collar or ring with the tide. Anchor moor refers to a stationary anchor system designed to keep the vessel in place, not accommodating vertical movement. Therefore, the spud's ability to adjust to tidal changes is why it is the correct answer.