OG Researchers now regard interferon

[AbeNeedsAnswers]

Researchers now regard interferon as not a single substance, but it is rather a biological family of complex molecules that play an important, though not entirely defined, role in the immune system.

A. as not a single substance, but it is rather a biological family of complex molecules that play
B. as not a single substance but as a biological family of complex molecules playing
C. not as a single substance but as a biological family of complex molecules that play
D. not to be a single substance but rather a biological family of complex molecules playing
E. not as a single substance but instead as being a biological family of complex molecules that play

C

[elias.latour.apex]

The core of the sentence involves the idiom not X but Y where X and Y are reasonably parallel. The original sentence is not parallel.

Only sentence (C) offers the parallel not as X but as Y.