imposing stricter limits should be modify the insurance only. The use of imposing modifies the whole phrase before it. That is needed here. So eliminate A and B.
D is wrong. impose...require...spending. According to the logic of the sentence, impose and require cannot be parallel.
C is wrong for the same reason.
I am going to go with E.

Here is the explanation from manhattan GMAT forum:

"
you have to realize which verbs are supposed to be parallel and which aren't. there's no grammatical formula for this; you have to examine the meaning of the sentence to figure it out.
- 'impose' (in whatever form) should be parallel to 'require' (again, in whatever form). these are two different things, both of which are aspects of the plan (= logical parallelism).
- 'spend' should not be parallel to 'see', because it functions as a modifier of 'see' (it's a descriptive adverb modifier, detailing the way in which the doctors see the patients).

choice a: 'spend' is ungrammatical here (it has no logical subject, and isn't parallel to anything).
choice b: imposing, requiring, and spending are all parallel, implying that the insurance plans do all three of these things (an absurdity in the last case).
choice c: all three verbs are parallel again, leading to the same absurdity witnessed in choice b.
choice d (= correct): the parallelism follows the model outlined above: only the verbs that are logically parallel appear in parallel structure.
choice e: 'requiring' and 'spending' are parallel in the modifier, implying that the plans themselves spend time with patients (in addition to requiring blah blah blah). this doesn't make sense."

So, choice D is correct.