Look at the right answer:

"Among lower-paid workers, union members are less likely than nonunion members to be enrolled in lower-end insurance plans that impose stricter limits on medical services and require doctors to see more patients, spending less time with each. "

Nested modifiers are modifiers that are modifying modifiers. Here "impose stricter limits on medical services and require doctors to see more patients" is modifying "plans". To determine this, ask "What is imposing stricter limits....?" The answer is "the plans are".

Now, at the end of the sentence there's ANOTHER modifier, "spending less time with each". This is modifying "doctors", which is a part of the first modifier. Hence, we have "nested modifiers".

Whenever I saw a present participle (i.e. spending) after a comma, I looked for the nearest noun that its modifying. My explanation gets a little fuzzy here, maybe others can bolster, but "patients" is too close to the modifier, so my guess is "spending less time with each" modifies the next nearest noun, "doctors", which makes logical sense.

In any case, by knowing "that" was required after "plans" (to indicate an essential modifier), and making the two verbs "impose" and "require" parallel and equal to each other in importance, you're lead to answer D anyway. No need to mess with the nested modifier!

Thanks Stacey and Cbenk.