Which of the following describes a groyne?

A groyne is specifically a structure built to protect the coast from erosion and to manage sediment deposition along shorelines. These structures extend from the shore into the water, typically constructed using wood, stone, or concrete, and are designed to interrupt the flow of water and reduce the energy of waves reaching the shore. As a result, groynes can help maintain beaches by accumulating sand on one side and preventing sand loss on the other. This coastal management strategy is vital for protecting coastal environments and human developments from the impacts of erosion and high-energy wave action.

The other options do not accurately describe a groyne: a long, narrow waterway could refer to a variety of waterways like canals or rivers but not specifically to a coastal structure; a type of rock formation refers to geological features rather than an engineering structure; and a natural stream or river describes water bodies that are naturally formed rather than human-made constructions aimed at coastal management.