For "Primary Purpose" questions, we should try to put the answer in our own words first, thinking about what the author ultimately wanted to convey with this passage.
Lines 38-40, to me, is the most valuable sentence in the passage. This is where the author introduces her opinion on prepaid legal services.
A) too neutral, not to mention there is almost no discussion of the traditional way of paying for legal services. But I would not be attracted to an answer beginning "compare and contrast" if I know the author staked out a specific opinion in the passage.
B) too neutral and too narrow (only really relates to the first paragraph or so)
C) the passage doesn't present and analyze statistics on how legal plans have changed prices, rather it speculates that continued use of such plans will have a downward effect on prices. Also, this is overly specific in terms of the author's concerns about legal plans (depressed prices was only one concern)
D) Sounds good. Sounds like the final paragraph, which was the author's point of view on the passage's topic.
E) The author expresses concerns about legal plans but does not suggest potential remedies.