Among lower- paid workers, union members are less likely than non union members to be enrolled in lower- end insurance plans imposing stricter limits on medical services and requiring doctors to see more patients, and spend less time with each.

(A) imposing stricter limits on medical services and requiring doctors to see more patients, and spend. Incorrect. "And spend less time with each" does not refer to doctors but rather the low end insurance plans..
(B) imposing stricter limits on medical services , requiring doctors to see more patients, and spending. Incorrect. "and spending" does not refer to doctors but rather the low end insurance plans.
(C) that impose stricter limits on medical services, require doctors to see more patients, and spend . Incorrect "And spend less time with each" does not refer clearly to doctors.
D) that impose stricter limits on medical services and require doctors to see more patients, spending - Correct. "And spend less time with each" needs to refer clearly to doctors. We need to maintain parallelism between "impose" and "require".
(E) that impose stricter limits on medical services, requiring doctors to see more patients and spending. Incorrect. There are two effets of low insurance plans - imposing strict limits and requiring doctors. This sentence tends to imply that the "requiring doctors" is an outcome of imposing stricter limits which is wrong.