https://www.beatthegmat.com/ing-modifier ... 43-45.html
The increased popularity and availability of televisions has led to the decline of regional dialects, language variations which originate from diverse ethnic and cultural heritages and perpetuated by geographic isolation.

(A) which originate from diverse ethnic and cultural heritages and perpetuated
(B) that originated from diverse ethnic and cultural heritages and perpetuated
(C) originated from diverse ethnic and cultural heritages and perpetuated
(D) originating from diverse ethnic and cultural heritages and perpetuated
(E) originating from diverse ethnic and cultural heritages and perpetuating
OA:D

<something> has led to the decline of X, language variations which A and B

The stuff after the comma is a modifier describing X (regional dialects)

The stuff following the which = two modifiers (indicated by the "and") that are both describing language variations, so A and B should be parallel.

One thing to know before we go any further. It is okay to have a present participle parallel to a past participle. That's still considered parallel because they are both participles.

So, in C, the "which A and B" part would read:
variations which originated from diverse ethnic and cultural heritages
and
(variations which) perpetuated by geographic isolation

That second one doesn't work as a sentence. You could say "which was perpetuated by geographic isolation." But you can't say "which perpetuated by geo. isolation."

In D, the equivalent part would read:
variations originating from diverse ethnic and cultural heritages
and
variations perpetuated by geographic isolation

each time, "variations" is followed by a participal. One is present and one is past, but that's okay - that can still be parallel.

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