The study, prepared for the county's economic development council, rep

definite A

Nothing wrong with the original; sentence is meant to emphasize the importance of the study by saying that it represents the most thorough attempt at X

Definite vs. indefinite article or noun modifier issue:
Eliminate C, D and E

Between A and B:
B describes the purpose of the study (no comparison).
A represents superlative comparison of the study among many same studies

A the most thorough attempt to gauge the impact (Hold it)
B the thorough attempt to gauge the impact
C a most thorough attempt to gauge the impact
D a thorough attempt to gauge the impact
E a most thorough gauge of the impact

Answer: A

The study, prepared for the county's economic development council, represents the most thorough attempt to gauge the impact of increased food and beverage taxes in the southeastern region since they were raised six years ago.

As per the meaning, Some study is the most thorough attempt (out of all attempts till now) to gauge the impact of something.

a)the most thorough attempt to gauge the impact
b)the thorough attempt to gauge the impact ( most is missing)
c)a most thorough attempt to gauge the impact ( "a most thorough" changes the meaning, now it conveys that there have been number of most thorough attempts, a scenario that is non-sensical)
d)a thorough attempt to gauge the impact ( the emphasis is missing with the omission of word most)
e)a most thorough gauge of the impact ( again , this choice communicates that there can be a number of thorough attempts)

If we take one take away from this problem, then that would be " The grammatical difference between "A" and "The" leads to a logical difference in the sentence"
while usage of THE MOST THOROUGH ATTEMPT .....emphasis on the pecularity of the study, The usage of A THOROUGH ATTEMPT.........doesn't lay that much of emphasis
Hope that makes sense

Isn't 'most thorough' repetitive?

No. Because "thoroughness" can have varying degrees. We can have "less thorough" and "more thorough" and we can therefore definitely have "most thorough"

Most => used as Adverb for adjective
Thorough =>adjective

A and D both seem correct to me. Can an expert please explain how D is wrong?

Kudos for good explanation(s)

If there is nothing wrong in the original sentence, mark it as a correct answer.

I believe D is wrong because the study doesn't "represent" a thorough attempt, it "was" a thorough attempt.

Isnt that valid for "It was the most throught attmept.." also?

Yes. Both would work: "it was the most thorough attempt" or "it was a thorough attempt" but since "represents" isn't part of the underline and we can only choose the answer that fits with "represents" I think the "study" can only represent a position like "the most thorough" but it can't represent an attempt it is/was an attempt.

Why the use of superlative 'most' is important here?

D is more concise than A. Not sure why it is wrong.

A. CORRECT. Here "thorough" is used to mean extensive/careful and like any other adjective, the superlative form is used to indicate that the "study" is the most extensive (i.e. most thorough) one among all the attempts in the last six years.

“The” is correctly used before the superlative form.

To depict an intention, the infinitive form “to gauge” is used.

Grammatical Note:

The words "most" and "thorough" are not redundant. Following is the explanation:

Any adjective has 3 forms: positive, comparative and superlative. Example:

Type 1A (regular - add er/est): Pretty, prettier, prettiest.

Type 1B (regular - add more/most): beautiful, more beautiful, most beautiful.

Type 2 (irregular - no definite rule): Good, better, best.

The adjective "thorough" is of type 2. The superlative form is thus "most thorough". It is intended to convey that of all the attempts, "the study, prepared for the county's economic development council" is the MOST thorough (superlative form).

B. The use of “the” without the superlative form “most” implies that the “study” is THE (only) “thorough” attempt , whereas all other attempts in the last six years are not thorough. This implication distorts the meaning.

Note:

Compare the following example:

I. “I have made an intelligent choice.”

II. “I have made the most intelligent choice.”

III. “I have made the intelligent choice.”

Statement I implies that there are many intelligent choices - I made one of them.

Statement II implies that there are many intelligent choices - I made the one that is the most intelligent one.

Statement III implies that there is only one intelligent choice and I made that choice.

In option B, the usage is similar to that in statement III above and hence the meaning is distorted.

C. Grammatically, the article “the” is mandatory before a superlative adjective. This use of “a” before the superlative adjective “most thorough” is wrong.

D. To depict an intention, the infinitive form “to gauge” is required to be used. In this option “for gauging” is wrongly used to depict an intention.

E. To depict an intention, the infinitive form “to gauge” is required to be used. In this option “for gauging” is wrongly used to depict an intention.
The correct answer is A

Definitely A

The sentence tries to say that the attempt is the "most thorough" and the emphasis is on the word "most"
Option D takes away that emphasis.

Nothing wrong with the original; sentence is meant to emphasize the importance of the study by saying that it represents the most thorough attempt at X