Sentence Correction

Although at one time children’s programming had been limited to a few hours a week, now, however, there are at least four cable networks that dedicate themselves entirely to shows produced specifically for children.

A. Although at one time children’s programming had been limited to a few hours a week, now, however, there are at least four cable networks that dedicate themselves
B. With children’s programming having been limited at one time to a few hours a week, now at least four cable networks have dedicated themselves
C. Once limited to a few hours a week, there are now at least four cable networks with children’s programming that dedicate themselves
D. Children’s programming was once limited to a few hours a week, but now there are at least four cable networks dedicated
E. While children’s programming had once been limited to a few hours a week, at least four cable networks are now dedicated

(D)

I don't even know the rule being tested here =(

I doubt OA here.....why do we need to have past perfect here....IMO we should have simple past here....

Moreover OG specifically say that one time and now cannot be parallel...

now....once = parallel.
one time...current time..... = parallel.

There is one OG question that specifically test this now/once rule.

Not sure how A is correct here.

IMO -D
I don't think A as correct answer

Same opinion. It should be D.
@doclkk : pls quote the source

I don't see any need to use past perfect tense, so A and E are wrong.
With one's doing is almost always wrong, so B is out.
once limited... there are is awkward, so C is out.
D is the right answer.

The had been in option A is unnecessary. Other options excluding D have wrong modifier. Thus D seems to be the best of the lot