Very good question. I couldn't able to solve, did some googling, and following is a thread from TestMagic:

https://www.urch.com/forums/gmat-sentenc ... nkite.html

Experts???

Rashmi1804 wrote: No less an authority than Walter Cronkite has reported that half of all Americans never read a book.
(A) No less an authority than
(B) Nonetheless an authority
(C) Nevertheless authoritarian
(D) Not less an authority than
(E) An authority not less than

What is the difference between " no less an authority " and "an authority not less " ?? why is the latter superior ??

thanks

What is the OA? I pick A.

As for your question, I believe the answer is that "no less an authority than" is a very old English idiom. I have seen it used many times in books and media articles. Never in my life have I seen or heard the idiom "an authority not less than".

Thanks!!

OA: A

No less an authority than Miss Universe has visited Guantánamo Bay and pronounced the infamous US detention centre a "relaxing, calm, beautiful place
- appeared in guardian.co.uk

"authority" as in the "in charge of " and not the "person" .
Who else other than Miss Universe can determine that a place is calm and beautiful -- appeals to the readership.
None other than Miss Universe has visited Guanta... and commented.

In a asimilar tone:
Walter Cronkite has reported that half of all Americans never read a book
is the message but to emphasise that Walter is fully qualified to give such a acomment, we have the idiom
No less an authority than Walter...
that is understanding .. any takers?