Never before had the musicians faced so many challenges at once as the

Never before had the musicians faced so many challenges at once as they had in playing Schubert’s Trout Variations.

A. so many challenges at once as they had in
Incorrect. The phrase "as they had in" is unnecessary.

B. at once as many challenges as
Incorrect. Placement of "at once" disrupts the flow of the sentence and makes it confusing.

C. at once as many challenges that they were when
Grammatically and logically incorrect

D. as many challenges at once as they confronted in
Idiomatically and grammatically correct

E. so many challenges at once that confronted them in
Incorrect. The phrase "that confronted them in" is awkward and disrupts the flow of the sentence