Visualization of mental images is a central concept in the Bates method—something not only which could achieve a state of inner balance and relaxation, but improving focus, eye coordination, blood circulation, and eye movement control.

(A) something not only which could achieve a state of inner balance and relaxation, but improving
(B) something which not only could achieve a state of inner balance and relaxation, but also to improve
(C) something which could not only achieve a state of inner balance and relaxation, but also improve
(D)that being something which could not only achieve a state of inner balance and relaxation, but also improving
(E) being something not only which could achieve a state of inner balance and relaxation, but improve

source - CrackVerbal
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Simple strategy:
“Once you’ve eliminated the impossible, whatever remains, however improbable, must be the truth.”

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Reading comprehension | Critical Reasoning | Absolute Phrases | Subjunctive Mood

Visualization of mental images is a central concept in the Bates method—something not only which could achieve a state of inner balance and relaxation, but improving focus, eye coordination, blood circulation, and eye movement control.

(A) something not only which could achieve a state of inner balance and relaxation, but improving
--Not only X but also Y is the correct idiom where X and Y should be parallel.
--Not only X but Y denotes contrast. The intended meaning here is not to show contrast

(B) something which not only could achieve a state of inner balance and relaxation, but also to improve
--Not only X but also Y is the correct idiom where X and Y should be parallel.
--X = could achieve ..... Y = to improve. These 2 are not parallel.

(C) something which could not only achieve a state of inner balance and relaxation, but also improve
CORRECT

(D)that being something which could not only achieve a state of inner balance and relaxation, but also improving
--"that being" has no role here.
--Not only X but also Y is the correct idiom where X and Y should be parallel.
--X = achieve ..... Y = improving. These 2 are not parallel.

(E) being something not only which could achieve a state of inner balance and relaxation, but improve
--"being" has no role here. It does not add any value
--Not only X but also Y is the correct idiom where X and Y should be parallel.
--X = which could achieve Y = improve. NOT parallel.
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Answer C - Used Not only and but also parallelism and only this one suits.

(C) something which could not only achieve a state of inner balance and relaxation, but also improve.

ADE fail this test. Also between B&C, B fails the parallelism ..