A. who had exercised thrice a week and who had slept eight hours a day feel
"feel" looked tempting because when speaking of historical events, laws or studies that still influences our present day, one may use present tense but here looking at the tenses gives you an illogical timeline; so first people exercised and slept (these acts are complete now), than study indicated that the same very people generally feels happy
B. that had exercised thrice a week while sleeping eight hours a day feel
you can't use "that" with people you have to use "who". Also the meaning is off. exercised while sleeping? no, what the author wants to say is that exercised + slept well
C. who exercised thrice a week and those who slept eight hours a day felt
CorrectD. who exercised thrice a week while sleeping eight hours a day feel
the meaning is off. exercised while sleeping? no, what the author wants to say is that exercised + slept well
E. that exercise thrice a week and sleep eight hours a day feel
you can't use "that" with people you have to use "who"