This is a tricky one.
But first, let's see the basics:
You should use THAT when you have to define or provide important information about a noun. Without that information you cannot identify the noun. For example:
I want to apply to a business school THAT ranks as one of the best schools in the world.
But if you can identify the noun, THAT is not required. In that case, you could use ", which". For example:
I want to apply to Harvard Business School, WHICH ranks as one of the best schools in the world.
You can see that "ranks....world" is just additional information. If you read only "I want to apply to Harvard Business School", you can identify the school. More information is not required.
Now let's see the question:
If we only read "the lake's ecosystem is one", we can see that it would be wrong. "ONE" has to be defined; we need THAT.
But, why can option E be correct if it doesn't have THAT? And here comes the trick:
We can ommit THAT when the object of the verb of the clause that defines (in this case, "they believe...") is the noun that it is being defined.
That's why B could also be correct, but it is wordy, and E is concise. So, E is better and correct.
I think you should read the MGMAT SC
book if you are begining to study SC. It's big, but it is good!
PS. Please, if someone disagrees, please reply.
Thank you for your detail explanation. I picked B and got it wrong