The largest trade-book publisher in the United States has announced th

GMAT shows a reasonably high tolerance for pronoun ambiguity. In any case, with E you should not worry about this because with E, the sentence would be:

The largest trade-book publisher in the US has announced the creation of a new digital imprint division, under which it will publish about 20 purely digital works, and it will sell them online as either electronic books or downloadable copies that can be printed upon purchase.

So, from a parallelism perspective, The largest trade-book publisher in the US has announced the creation of a new digital imprint division, under which two things will be done:
(i) it will publish about 20 purely digital works, and
(ii) it will sell them online as either electronic books or downloadable copies that can be printed upon purchase

Notice that it in (i) is clearly acceptable to GMAT (because it's in the non-underlined portion:)). So, clearly, it in (i) is referring to largest trade-book publisher in the US. Hence, from a parallelism perspective, it in (ii) will also correctly refer to largest trade-book publisher in the US.

By the way, this is an excellent question where the usage of passive voice is preferred, because the original sentence just says that the digital works are to be sold online. The original sentence does not specifically tell us as to who will sell them online. So, C and E are changing the meaning of the sentence, by mentioning that the publisher will sell them online. Quite subtle.

p.s. Our book EducationAisle Sentence Correction Nirvana discusses this aspect of passive voice, its application and examples in significant detail. If someone is interested, PM me your email-id, I can mail the corresponding section.