Official Explanation:
E) This question tests parallelism and clarity of expression. In its proper parallel form, this sentence would read: Viruses, due to their inability to sustain life processes on their own, are not classed among living organisms, but among parasites. Because none of the choices offers this construction, you must find the choice that comes closest without changing the meaning of the sentence or introducing new errors.
Choice (C) attempts to correct one error by introducing the word being. However, this choice changes the meaning of the sentence. The original sentence is trying to clarify how viruses are classified, not what they are (a small but significant difference). This choice implies that viruses are parasites and uses "between" instead of "among," which is incorrect in this sentence.
Choice (D) makes the same errors as choice (C) (Viruses are parasites and "between").
Of the remaining choices, (E) comes closest to replicating the original meaning of the sentence. The clause viruses are classed among parasites, not among living organisms clearly explains how viruses are classified, and the phrase unable to sustain life processes on their own clearly modifies "parasites".
Choices (A) and (B) are close to being correct, but they use "between" instead of "among" for "between parasites" in choice (A) and "between living organisms" in choice (B).
Choice (E) does the best job of clarifying the meaning of the sentence, and it introduces no other errors. Choice (E) is the best answer.