I would go with A as well
no less than someone/something - this very important person or organization
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
No less an authority means "who else could be better than Walter Cronkite
Nonetheless and nevertheless mean the same - however which quite does not sound right. So eliminate B and C.
If you were to use E then authority is compared to Walter Cronkite. But instead it needs to be compared to Walter Cronkite's authority... Hence the Sentence should read "An authority not less than that of Walter Kronkite
D sounds the same way as E. Authority is compared to Walter Cronkite more than his drawing the comparison between the authority and Walter cronkite's authority.
Hence A is the best answer. Hope I am right.