Final solution at one place:

Important: The purpose of this post (and all the other posts by me) is to give a complete solution to all GMAT-Prep Verbal questions at one place. Sometimes students have to wade through dozens of posts to get to the final answer. My posts will give one complete and crisp solution required to arrive at the correct answer by eliminating the wrong one. Some of the content in these posts may have been taken from various other sources (discussion forums).

Compared to those who do not, running three days a week saves an average of 14.1 percent on health-care costs each year in the United States.
(A) Compared to those who do not, running three days a week saves
(B) Compared with nonrunners, people who run three days a week save
(C) Compared to not running, those running for three days a week save
(D) When compared with those who do not, people running three days a week save
(E) When compared to nonrunners, running three days a week saves

(A) isn't parallel: it compares people ("those who do not...") with running itself.

(C) isn't parallel: it compares "not running" with people ("those running for...")

Remember: The use of WHEN is always wrong in such a situation. It means only at that moment when the comparison is made will people save money ... absurd!

(D) and (E) misuse "when". You cannot use "when" unless you are literally talking about the timeframe in which an event takes place. According to (D) and (E), these savings are only realized when someone is actually making the comparison. (i.e., only when someone else is looking at the statistics, comparing them, do these people actually save money!).

(E) is also not parallel: it compares people "non-runners" with running itself.

B: Correct