Compare "which evidence," "evidences," "an evidence," and "evidence" by itself. Which form sounds most natural in formal English when talking about scientific proof or data?

Look at the verbs in the part about what the researchers believed. Should that verb be in present tense or past tense, given that it refers to a belief held in the past?

Cross out any choice that has even one clear error (wrong noun form, wrong tense, awkward or ungrammatical structure after the comma). The remaining option should read smoothly when you insert it into the sentence.

First, notice that the main sentence is already complete: "Shedding light on the evolution of cephalopod vision, a newly discovered fossil squid has preserved traces of its optic nerve, _____." After the comma, we do not need another full sentence; we need a phrase that adds extra information about those traces (what they show or suggest). So the blank should be filled by a phrase that continues the idea smoothly, not by something that introduces a new subject and verb in an awkward way.

Try each option after the comma:

• "which evidence suggests that ..." starts with which plus a noun, which is not a normal or clear way to continue from the comma. After a comma, we would expect either "which suggests" (with which referring to a previous noun) or a straight noun phrase, not "which evidence...".
• The other options begin directly with "evidence" or "an evidence" or "evidences," which are all noun phrases that can function as an appositive (extra information) after the comma. So we should focus next on which noun form is correct and natural.

In standard English, "evidence" is usually an uncountable noun when we talk about information or proof. That means we normally say "evidence" alone, not "evidences" and not "an evidence" in this context.

• A choice that uses "evidences" is incorrect here because the plural is not used in this general sense on the SAT.
• A choice that uses "an evidence" is also incorrect because we do not usually use an article with uncountable nouns.

So we should keep only the option where "evidence" appears by itself, without an and without being pluralized.

The phrase inside the blank describes two past ideas:

1. When complex eyes actually evolved.
2. What researchers used to believe about that timing.

We want simple, clear past tense for both: something like "evolved" and "once believed" both refer to past events, with the belief happening in the past and later being revised. Any answer that says something like "once believe" (present tense) does not match this past-time context and is ungrammatical.

The only choice that:

• Uses "evidence" correctly as an uncountable noun (no an, no plural evidences),
• Fits smoothly after the comma as an appositive phrase describing the traces of the optic nerve,
• And uses consistent past-tense verbs ("evolved" and "once believed"),

is "evidence suggesting that complex eyes evolved at least 200 million years earlier than researchers once believed."