leeye84
syamee_u
(C) - parallel
"that" is required as plans are imposing stricter limits.
My bet goes to E.
The question does test parallelism, but C does not work because:
insurance plans that impose limits (ok)...
insurance plans that require doctors to see more patients (ok)...
insurance plan spend less time with each (not ok).
Doctors see more patients and spend less time with each.
Hopefully it helps.

I also went with C and then realized that by imposing the stricter limits the plan is causing doctors see more patients and subsequently cutting down time the doctor is spending with the patient, this logic makes the answer E.
check out the SC questions 1-70 document in this link:
https://www.gmatclub.com/phpbb/viewtopic.php?t=34300
note question 21, it is the exact same question, only answer choices D & E are different, and the OA is D.
I'm surprised that someone posted this question without verifying if the choices were correct.
It can be quite detrimental working on questions which do not have valid sources.