A decrease in semantic association and an increase in frontal lobe act

'Suggests' is the main verb of the sentence, the subject is ' A decrease in semantic... and an increase in frontal....', so the verb should be in plural form. Hence
A and B are out - SV error
C and D are missing the main verb 'are' if using present continuous tense
C is correct

Check the Original sentence carefully, 2 actions are discussed -

1. A decrease in semantic association
2. An increase in frontal lobe activity

As such plural verb Suggest is correct, further here, the verb “suggest” means to provide an idea or convey a thought to be considered.

Official Explanation:

Since the underlined portion of the sentence contains the main verb of the sentence, suggests, check that it agrees with the subject. The subject of the sentence, a decrease…and an increase, is plural. Since the verb suggests is singular, there is a subject-verb agreement error. Eliminate choice A and look for obvious repeaters. Choice B is an obvious repeater since it begins with the singular verb suggests, so eliminate choice B. Now, evaluate the remaining answer choices individually looking for reasons to eliminate each.

Choice C fixes the original subject-verb agreement error by using the plural form of the verb suggest, and it introduces no new errors, so keep choice C. Choice D changes the verb tense to the participle suggesting, which creates a sentence fragment in a sentence that requires the simple present tense verb suggest, so this is a verb tense error. Eliminate choice D. Choice E also incorrectly changes the verb to suggesting, so eliminate choice E.

Choice A: No. The plural subject a decrease…and an increase does not agree with the singular verb suggests. Subject-Verb agreement.

Choice B: No. The plural subject a decrease…and an increase does not agree with the singular verb suggests. Subject-Verb agreement.

Choice C: Correct.

Choice D: No. The verb suggesting is incorrect in a sentence that requires the present tense verb suggest. Verb tense.

Choice E: No. The verb suggesting is incorrect in a sentence that requires the present tense verb suggest. Verb tense.

The correct answer is choice C.

Thanks Abhishek009 for the explanation

Thanks Starfruit for the explanation

A. suggests that a human's memories might not be as well-organized as some neuroscientists once thought.

1. Decrease in X and Increase in Y (compound subject) + suggests (singular verb) ----- Incorrect.

B. suggests that a human's memories might not be so organized as some neuroscientists once thought.

1. Decrease in X and Increase in Y (compound subject) + suggests (singular verb) ----- Incorrect.

2. Use of "so.....as" changes the meaning.

C. suggest that a human's memories might not be as well-organized as some neuroscientists once thought.

1. Decrease in X and Increase in Y (compound subject) + suggest (plural verb) ----- CORRECT

D. suggesting about a human's memories that they might not be so well-organized as once thought by some neuroscientists.

1."Suggesting" is modifying "something obscure". This means "Something obscure" is suggesting about human's memories. ----- INCORRECT.

2. Use of "so.....as" changes the meaning.

3. Use of "they" is incorrect.

E. suggesting a human's memories might not be as well-organized as once thought to be by some neuroscientists.

1."Suggesting" is modifying "something obscure". This means "Something obscure" is suggesting about human's memories. ----- INCORRECT.

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