Which zone is defined as the area between the high-tide and low-tide mark?

The intertidal zone is the correct choice as it specifically describes the area that lies between the high-tide and low-tide marks. This zone experiences regular and significant fluctuations in environmental conditions due to tidal movements, which lead to varying exposure to air and water. Organisms in this area must be adapted to both submerged conditions during high tide and desiccation during low tide, leading to a unique and diverse ecosystem.

The other options represent different marine environments. The benthos refers to the bottom layer of a body of water, which includes organisms that live on or near the sea floor. The neritic zone pertains to shallow waters found near shore and extending to the continental shelf, where sunlight penetrates and allows for photosynthesis. An estuary is a partially enclosed coastal area where freshwater from rivers meets and mixes with saltwater from the ocean. These areas, while crucial to marine biodiversity, do not define the specific tidal characteristics associated with the intertidal zone.