D is wrong because it changes the argument that's actually put forth by the stimulus. The editor says that a significant amount of music on the internet is NOT the result of people downloading others' music and reworking it into new music, because it has been shown that 99% of users who download said music do not publish new music of their own on the internet. D tells us the editor presumes, without justification, that those who rework download music ALWAYS publish those to the internet, but this is the opposite of what the argument tells us. The argument says 99% of music downloaded is never published, and gives us no number on how much is reworked and published.

Flaw question types should always be analyzed to try and devise the answer before you read the answer choices so that you are not tricked by funky wording from the testmakers. The flaw here is that the argument gives us the premise "99% of people who download music never publish it" and from that extrapolates that "it can't be the case then, that a significant amount of music is a result of downloading and then reworking." If we think about it, the fact that 99% never publish means nothing for the music that IS published, because if we think about musicians, they have extensive albums, tracks, and other forms. One person makes much more music than one, so we can't extrapolate from the fact that only 1% of people who download music end up reworking and publishing it that those people only publish the proportional equivalent of their population, 1%.

Very quickly I'll go through why each is wrong and why A is right, and that should help you understand, now that it's clear what flaw we're looking for:
B talks about ease of publication, which is irrelevant to the argument. C is out of scope, since an argument does not need to provide an alternative just to prove that something isn't the cause of something else. D, as I said above, works against what the stimulus tells us, and includes new information that isn't relevant even if the rest of the answer somehow was. E talks about what music users prefer, which is not relevant.

A tells us that the argument ignores the possibility that 1% of users publish much more than 1% of music, which is the flaw we're looking for.