Among lower-paid workers, union members are less likely than

First, we can determine that "that impose" is necessary and eliminate A&B. That leaves us with C,D, and E.

C is incorrect because it incorrectly uses parallelism for the series. While the verbs used are correct, the way the verbs are used changes the meaning of the sentence.
In C, the logic follows this : Insurance plans impose, require and spend. Well, that's not what the original sentence means. The doctor sepend less time, not the insurance plans. Throw out C.

Because I think D is correct, I'll go to E.

E uses "requiring doctors". The entire sentence is awkward. If it is supposed to be parallel, it fails because the verbs are not parallel. If the "requiring doctors" is supposed to show some of the stricter limits, the sentence is incorrect as it should say "that impose stricter limits on medical services, which require doctors to see..."

D is correct for proper use of parallel verbs. "insurance plans that impose...and require..." This is parallel and the meaning of the sentence is the same as the original.