Though all competitive exams do not allow using a calculator, but where they are permitted, there are restrictions on the models allowed.

A. Though all competitive exams do not allow using a calculator, This implies that no competitive exam allows a calculator in contradiction with the rest of the sentence. Eliminate.

B. Though all competitive exams do not allow calculators, Same error as in (A). Eliminate

C. Though using the calculators is not allowed in all competitive exams, "the calculators" incorrectly implies that only a given set of calculators is being referenced. Eliminate.

D. Not all competitive exams allow the use of a calculator, Correct answer correcting the above errors.

E. Every competitive exam does not allow using of calculators, "using of calculators" is incorrect. The correct structure would be "usage of calculators". Eliminate.

Hope this helps.
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D looks correct but have dout regarding
Not all competitive exams allow the use of a calculator, but where they are permitted, there are restrictions on the models allowed.
Calculator and they (pronoun for calculator) mismatches
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I doubt that you would ever find a question like this on the GMAT, as all 4 wrong answer choices have the same mistake. This would be a good example of a non-idiomatic use of conjunctions as well as a sentence which contradicts itself.

Though all competitive exams do not allow using a calculator, but where they are permitted, there are restrictions on the models allowed.

A. Though all competitive exams do not allow using a calculator, FALSE
1. The original sentence begins with though which implies that a contrast is coming. Following this with "but" which in itself implies a contrast is non-idiomatic and always wrong. The sentence is implying a contrast to an already implied contrast.
2. The sentence is also illogical as it explains that a calculator is never allowed in competitive exams. Then the sentence goes on to contradict itself by saying "but where they are permitted" implying that the extreme of the word "never" is false.

B. Though all competitive exams do not allow calculators, FALSE
Repeats mistakes 1&2

C. Though using the calculators is not allowed in all competitive exams, FALSE
Repeats mistakes 1&2

D. Not all competitive exams allow the use of a calculator, CORRECT
Mistake 1 is corrected by removing the word "though" and never removing the double contrast issue.
This sentence also solves mistake 2 as it has a different meaning. It does not say calculators are never allowed. Instead, the sentence explains that "not all" exams allow a calculator. This does not exclude that some exams may allow a calculator

E. Every competitive exam does not allow using of calculators, FALSE
This answer choices did not make mistake 1 as it removed the word though, however the sentence replaces though with "every". This word is an extreme word and leads to the second mistake. The first half of the sentence states that something never happens while the second half contradicts this.

Same doubt. I think they in clause started with "but" is used to refer calculator, which needs to be plural but here it is singular(in the correct answer).

Anyone please explain

I first liked D, but when aI saw they referring to calculator, it spoiled everything. is this usage correct?

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