The ocean has the greatest number of ecosystems. The ocean is a vast and diverse environment that is home to a wide range of ecosystems, including coral reefs, kelp forests, estuaries, and deep-sea environments. These ecosystems are home to a diverse array of plant and animal life, and they play important roles in the earth's biosphere. The other options, river, forest, and grassland, also contain a large number of ecosystems, but the ocean has the greatest number due to its vast size and diversity.