How will the right answer fit in terms of support and conclusion?

Only the right answer will be support that guarantees the conclusion.

Highlight the main conclusion in the passage, if there is one:

there must not be a thriving population of turtles at Wallakim Pond.

[SUPPORT]. [SUPPORT]. Thus, [CONCLUSION].

The author changes the subject between the first and second piece of support. The implication is that Wallakim Pond fails the rule in the first sentence about “beneficial” conditions because it’s “acidic”. But the author doesn’t explicitly make that connection, so the right answer will have to.

Map the wording of the answers to the wording of the passage:

(A) If the water in a pond is not acidic…

…then that pond is in a different passage. The only pond in the argument is “acidic”, so this is outside the scope and doesn’t support the conclusion.

(B) The most important factor…

…is never mentioned. The argument doesn’t compare the importance of different factors. Acidity is the only factor mentioned at all.

(C) The water conditions at Sosachi Pond are more beneficial…

Cool, so I’ll know where to take my pet turtle for her next swim. But of course, info about any other pond can’t really support a conclusion that’s only about Wallakim Pond.

(D) …would have a thriving population of turtles if it were not acidic.

Sure, and I’d live forever if I wasn’t gonna die someday. This answer is about a hypothetical condition the passage says isn’t true. Who cares what would happen “if it were not acidic”?

(E) …beneficial only if the water in the pond is not acidic.

This is the same as saying, ‘if the water is acidic, then the conditions are not beneficial’. That maps to the wording in the passage and matches the author’s implication exactly.

(E) is the correct answer.