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 Incorrect comparison between people ("those") and running. Eliminate.
B. Compared with nonrunners, people who run three days a week save Seems to be the correct answer except for the use of "nonrunners". Hold on to this option for the time being.
C. Compared to not running, those running for three days a week save Incorrect comparison between not running and people ("those"). Eliminate.
D. When compared with those who do not, people running three days a week save Incorrect use of "when" - a time marker. Also, when omitting a verb (ellipsis) such as in "who do not," ("run" is omitted), the verb should have been used earlier in the sentence. Eliminate.
E. When compared to nonrunners, running three days a week saves Incorrect comparison between people ("nonrunners") and running. Also, incorrect use of "when" - a time marker. Eliminate.
Given all other options have clear errors, we can go with option (B).
Hope this helps.