First of all we have the classic "not X but Y" parallelism. The second part is fixed as "from their role", so we need a "from" phrase in the first part. (A) & (B) & (C) are out. That leaves (D) & (E).
New, we need parallelism.
(D) has "from their importance" in parallel with "from their role" --- noun to noun, a perfect parallel construction.
(E) has "from being important sources ..." in parallel with "from their role" ---- a verb-form, a gerund, set up in parallel with a noun --- in GMAT SC terms, this is an abysmal failure of parallelism. Parallel structure might be the single most tested concept on the GMAT SC, and they are sticklers for having exactly two of the same thing in parallel. That's why (E) is a trainwreck failure and incorrect.
Answer = D.
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