Answer:
D. the open ocean
Explanation:
The reason why I cannot be any of the other answers are because, A is just a single organism which would in fact be incorrect. A school of fish would be incorrect because, it only includes a bunch of the same fish. The Ecosystem (As it says in the capture below) is a bunch of different organisms that work together like a shark might eat the fish but, that is about it. The entire ocean is a great example of a ecosystem because, there are all kinds of things that create the ecosystem. Therefore, your answer could only be D. the open ocean.
Information in the picture below:
