The question states that "no less than a technical and scientific breakthrough is necessary". No less than here implies that whatever their champions do should be atleast equal to a technical and scientific breakthrough which is what option C states but in a more concise way.

Hence C.

Сorrect !

this question is from OG 10th. #242.

Explanations:
C is the best choice. The word that functions grammatically to introduce the clause that describes the point that champions of solar cells concede. Choices A and B needlessly lengthen the statement by expressing the idea through negation: no less than and nothing other than could be dropped without loss of meaning. In D and E, the preposition/or is less idiomatic than o/in expressing necessity. Furthermore, both choices present an awkward and wordy noun-plus-prepositional phrase instead of a that clause that would express meaning more exactly and concisely.

One thing what I du not understand is it necessary to use "for" aftrer the word "necessary" in C?

thanks
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Its C !!

Reasoning as follows :

first deconstruct the sentence in its smaller dependent clauses or independent clauses

Dependent Clause 1 - Even their most ardent champions concede < Subject is their, Verb is Conceded >

Dependent Clause 2 - that no less < that subject ...Verb - absent >

Dependent Clause 3 - than a technical or scientific breakthrough is necessary[/u] before solar cells can meet the goal of providing one percent of the nation’s energy needs. < Subject - A technical & scientific breakthrough, Verb - is necessary >

Now Structure of Complete sentence is DC1+DC2+DC3 = 1 IC

In this DC2 : lacks verb because of which it is wrong in A & B

In C : 2 Dc are connected by that , to make 1 IC, thus most appropiate andwer.

Tip : In case of confusion, always deconstruct sentence into its main & sub clauses.

Regards
ST
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Is the use of "No less than..." always wordy, thus wrong?

C is simple and perfect. Can anyone please explain, Are other options grammatically sound?
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(A) that no less than a technical or scientific breakthrough is necessary
(B) that nothing other than a technical or scientific breakthrough is needed
(C) that a technical or scientific breakthrough is necessary

(A) No less than 100$ is needed
(B) Nothing other than 100$ is needed
(C) 100$ is needed

I think, (B) and (C) convey the same meaning, where as (A) conveys different meaning (anything >= 100$ is fine)

Thanks.
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Once you know the answer, it is easy to justify.

Technically there is nothing wrong with C but it changes the meaning of the sentence.
Option A says that a minimum of 'a technical or Scientific breakthrough' is required before solar cells. But Option C only says that 'a technical or Scientific breakthrough' is necessary.

Feel free to correct me if I am wrong, but less, which is used for uncountable objects, is used correctly here IMO.
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Press +1 if my explanation helps. Thanks!