Between C and D, but I opt for D:
The decline of X , originating from something & perpetuated by...

Could you please provide a reason for it?

eitijan wrote: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.

The answer choices are all a little weird, but I guess one of them is the best.

(A) which originate from diverse ethnic and cultural heritages and perpetuated

Restrictive modifier, which originate, is incorrect. Has to start with that.

which (or that) ... perpetuated by does not make sense. It's half active and half passive. To make sense, has to be passive form that ... are perpetuated by.

(B) that originated from diverse ethnic and cultural heritages and perpetuated

Repeats the that ... perpetuated by error. Has to be that ... are perpetuated by.

(C) originated from diverse ethnic and cultural heritages and perpetuated

This almost works as now we have two past participle or verb -ed modifiers, originated and perpetuated by. There is, however, a somewhat subtle meaning issue, in that things are generally considered to originate themselves, rather than to be originated. In other words the dialects were not originated by someone. They originated. So the version created using this answer choice does not make sense as it conveys that the dialects were originated rather than that they originated.

(D) originating from diverse ethnic and cultural heritages and perpetuated

This one is a little funny as it conveys that the dialects are still originating, but maybe they are. So I guess we can go with this choice. Simplifying the modifier we get, language variations originating from heritages and perpetuated by isolation. That works.

(E) originating from diverse ethnic and cultural heritages and perpetuating

language variations perpetuating by isolation does not make sense.

So the best choice is D.