Paleontologists studying the fossil record have estimated that the current collection of known fossils accounts for only approximately 10% of all species that have ever existed.

Meaning:

1. There is paleontologists who study the fossil record
2. They have estimated that the current collection of known fossils accounts for only (approximately) 10% of all species that have ever existed

Errors:


v-ing right after the noun without comma 'studying' correctly modifies "paleontologists"
all SV pairs are in place and agree in number

One strage thing to me is "only" and "approximately"

"approximately" - close to a particular number or time although not exactly that number or time:

In December, the company reduced its workforce by approximately 5%.

"only"


---> Perfect explanation "Understanding the Word “Only” (Mandatory to read): <---

Keep in mind “only” is a modifier. This means that it modifies what you are trying to say and acts as a descriptive word in your sentence. “Only” can have a strong presence in a sentence and can shift the meaning of a sentence drastically if it is used incorrectly.[1] To create a clear sentence, you should place “only” next to the noun, subject, verb, or phrase you are trying to modify.
When the word “only” is misplaced in a sentence, it becomes a “misplaced modifier”. This means the modifier, for example “only”, is not placed beside the noun or other word it is trying to modify. This creates confusion on the part of the listener or reader because the sentence becomes awkward... --->

So we have 10% of all species that have ever existed
'only' has to modify '10%' not 'approximately'

only 10% of all species that have ever existed -- correct
approximately 10% of all species that have ever existed -- correct
only approximately 10% of all species that have ever existed -- incorrect

POE

A. studying the fossil record have estimated that the current collection of known fossils accounts for only approximately 10% of all species that

B. that study the fossil record have estimated that the current collection of known fossils is only 10% of all species that
('that' - cannot modify people)

C. studying the fossil record have estimated that the current collection of known fossils accounts for only 10% of all species that

D. who study the fossil record have estimated that the fossils currently collected are only approximately 10% of all species which
(same as A, 'which' - needs to be preceded by comma)

E. who study the fossil record estimate that the current collection of known fossils accounts for approximately 10% of all species which
(paleontologists estimate - present simple - it is not a general fact or rule, we don't need this tense here, 'which' - needs to be preceded by comma )

C is the answer
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Official Explanation:

Since the underlined portion of the sentence contains the word approximately, check whether this word is consistent with the rest of the sentence. The words estimated and approximately are both used in the sentence, even though they have similar meanings and only one is needed. This is a redundancy error, so eliminate choice A and look for any obvious repeaters. Choice D is an obvious repeater since it contains the words estimated and approximately, so eliminate choice D. Now evaluate the remaining answer choices individually, looking for reasons to eliminate each.

Choice B fixes the redundancy error by removing approximately, but incorrectly uses the pronoun that (rather than who) to refer to paleontologists, so eliminate choice B. Choice C fixes the original redundancy error by removing the word approximately, and introduces no new errors, so keep choice C. Choice E uses both estimate and approximately, which is redundant, so eliminate choice E.

Choice A: No. The use of both estimated and approximately is redundant. Redundancy.

Choice B: No. The use of that to refer to paleontologists is incorrect. Pronoun agreement.

Choice C: Correct

Choice D: No. The use of both estimated and approximately is redundant. Redundancy.

Choice E: No. The use of both estimate and approximately is redundant. Redundancy.

The correct answer is choice C.
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