The correct answer is option 3: Estuary. Estuaries are highly productive ecosystems where freshwater from rivers and saltwater from the ocean mix. They have a high level of nutrients and are home to a diverse range of plant and animal species. Estuaries support a large amount of primary production, which is the process of converting energy from the sun into organic matter. This productivity is due to the combination of nutrients from rivers and the constant mixing of water. As a result, estuaries provide important nursery habitats for many marine species, including fish and shellfish. This high productivity makes estuaries the most productive ecosystems in the biosphere. Therefore, among the given options, the correct answer is option 3: Estuary.