IMO D

One thing is that rising here is incorrectly used, for this particular sentence, authorities will be worried about a rise in number of something, rising will be like its still rising
With this in mind, one can eliminate B and C

Now out of A D and E

A. being concerned about the rise in the number of car accidents and the average person’s attention span being shrunk gradually
They are not in a process of(being, generally means this) getting concerned, this not even parallel.

D. concerned about the rise in the number of car accidents and the gradual shrinkage of the average person’s attention span
They are concerned about two things
1) rise in the number of car accidents
2) the gradual shrinkage of the average person’s attention span

E. given that they had been concerned about the rise in the number of car accidents and the gradual shrinkage of the average person’s attention span as well
as well in the end breaks the parallelism.
_________________

past participle is the only one in option that can modify noun activist

So option D

concerned is that only past participle in option D that can work as a modifier to noun activist

So Ans is Option D

Not 100% on all of the below, especially the use of being

Public safety activists, being concerned about the rise in the number of car accidents and the average person???s attention span being shrunk gradually, feel that the introduction of the ???netcar,??? a vehicle equipped with access to the Internet, is a terrible mistake.

A. being concerned about the rise in the number of car accidents and the average person???s attention span being shrunk gradually
The first being is used as a modifier. It is not a progressive tense verb (not preceded by a form of to be), nor is it a gerund (not the subject or object of the sentence). GMAT does not favour its use as a modifier
the rise in the number of car accidents and the average person???s attention span being shrunk gradually are not parallel. the+noun+prepositional phrase containing noun vs the+possessive+noun+modifier

B. concerning themselves about rising car accidents and shrinking attention spans among average people
I think concerning is a hypercorrection here, trying to attain parallelism with rising and shrinking, but I can't quite say why it's wrong
rising car accidents[i] ??? car accidents aren't rising, rather the number should be

C. because they are concerned about the rising of the number of car accidents, also the shrinking of the attention span of the average person
[i]rising is an adjective and should be touching the noun number
also is not a conjunction

D. concerned about the rise in the number of car accidents and the gradual shrinkage of the average person???s attention span
Nice
the rise in the number of car accidents is the+noun+prepositional phrase
the gradual shrinkage of the average person???s attention span is the+adjective+noun+prepositional phrase
Parallelism is maintain

E. given that they had been concerned about the rise in the number of car accidents and the gradual shrinkage of the average person???s attention span as well
given that they had been is unnecessary, but I don't think it's grammatically/rhetorically/logically wrong otherwise
the rise in the number of car accidents the+noun+prepositional phrase is not parallel to the gradual shrinkage of the average person???s attention span the+adjective+noun+prepositional phrase containing a possessive
as well is redundant