Ans: C. Selection bias(Ref Park 24/e p78, 88, 23/e p73, 22/e p71; en.wikipedia.org/wiki/Bias_(statistics); http://sphweb. hume.hu.echdollt/mph-modules)Selection bias:Groups to be compared are differentially susceptible to outcome of interest, even before experimental maneuver is performed.Also referred as "Susceptibility bias".In the given question, researcher is concerned that the smokers might differ from non-smokers in their diet, exercise, etc.Hence, this concern is known as selection bias.