According to criminologists, the cause of riots among many

Can I choose none of the above?

If I had to pick one of the listed choices, (A) is the clear winner. However, the original sentence has parallelism issues that would make it an unlikely choice on the actual GMAT.

Reading the original sentence, we see "as that ..." in the non-underlined section following the underlined section. Based on that clue (remember, the non-underlined portions must be correct!), we know that the underlined portion has to be set up in a similar fashion.

In the original, we have:

"the cause... is not so much that... as that...", preserving parallelism.

(B) has "the cause... is not as much their... as much as that.." which isn't parallel - eliminate.

(C), (D) and (E) all start with "it is not so much...", which isn't parallel to what comes after the underlined portion - eliminate.

So, by default, (A) is the correct choice.

Another quick way to choose (A) is to note that (B), (C), (D) and (E) all introduce ambiguous pronouns ("they" and "their" don't have a clear referent, whereas "rioters" in (A) makes the reference clear).

However, (A) also isn't perfectly parallel.

The bolded parts of "is not so much that rioters are being primed for violent behaviour by their parents or to rebel against oppressive regimes by the media" are different forms of language.

Rioters are being primed for two things: "for violent behaviour" and "to rebel". To be truly parallel, the sentence should read "to behave violently" and "to rebel" or "for violent behaviour" and "for rebellion".