We have to consider that abiotic elements are those components of an ecosystem that are not alive, such as, for instance, the wind, the water, the sand, the sunlight, and the salt.
In this case, we know that organisms such as algae can be found in both the ocean and freshwater ecosystems, like rivers. The same happens with sand, which we can find at the bottom of both kinds of bodies of water, and also the sunlight; we know that it comes from the sun and irradiates all of our planet.
Having this clear, we can say that the abiotic factor that separates or differentiates marine ecosystems from river ecosystems is the salt (A).