It is a mistake to think, as ecologists once did

We have been asked to find the main conclusion of the argument.

Main conclusion of an argument is the author's final opinion and will be supported by surrounding evidence.

Let us read the paragraph provided to us --

"It is a mistake to think, as ecologists once did, that ..." -- seems to me that the author is trying to express an opinion. Let me read on. This is the author's opinion -- "mistake to think that natural selection will eventually result in organisms that will be perfectly adapted to their environments."

WHY does the author state that opinion? The reasoning is mentioned in the next sentence -- Individual's environment can vary tremendously; therefore, no single set of attributes could possibly prepare an organism to cope with all the conditions that it could face AND thus perfect adaptation of an individual to its environment is impossible.

Note that the second sentence CANNOT be the main conclusion of the argument because it gives a REASON for the author's rejection of ecologists' perception. (The reason being this -- environment can vary tremendously --> no single attribute can prepare an organism adequately --> perfect adaptation not possible. This leads to the main conclusion)

We have to look for an answer option that matches the first sentence of the paragraph -- Option B seems to be an exact match and hence the correct answer.

Option A - Incorrect
The conclusion states that "mistake to think that natural selection will eventually result in organisms that will be perfectly adapted to their environments".
It states that such a thing is not possible through natural selection. We DO NOT KNOW whether such a thing is entirely impossible.

Options C and D -- Incorrect
Are intermediate conclusions. Not the main conclusion.

Option E - Incorrect
Even though this might be correct, it is NOT the author's opinion BUT was once ecologists' opinion. Incorrect.