Which of the following is a type of nursery structure used in coral restoration?

A floating nursery structure is a commonly used method in coral restoration efforts. These structures are typically designed to keep coral fragments safe and facilitate their growth in a controlled environment. By being positioned at or near the surface of the water, floating nurseries benefit from adequate sunlight, which is essential for the growth of corals through photosynthesis. Additionally, these structures are easier to monitor and maintain, allowing for effective care and management of the coral fragments.

The other options, while they might seem plausible, do not represent standard types of nursery structures used in coral restoration. For example, submerged structures exist but are typically not categorized as 'nurseries' in the same way as floating ones, as they can face challenges in visibility and access for monitoring. Similarly, 'sinking' and 'burrowing' are not recognized methods in coral nursery setups. Thus, the floating nursery is the most effective and widely adopted option for promoting coral growth and restoration.