Ans is 'c' i.e. Confounding factor In an epidemiological study, in addition to the exposures that the study is investigating, there may be other factors that is associated with the exposure and independently affects the risk of developing disease. If the prevalence of these other factors differ between groups being compared, they will distort the observed association between the disease and exposure under study. These distorting factors are called confounding factors or variables. In the study given, protective factor is being studied. But as high fibre content of vegetarian food is protective against CA colon, it can confound or distort the observed protective role of b-carotene present in vegetarian food. Thus this is an example of confounding factor. Causal association: There are 2 types- 1. One to one causal relationship - Two variables are causally related if a change in one variable is followed by a change in the other. So when the disease is present, the factor must also be present. In the given study causative factor is not being studied rather preventive factor is being studied.Multifactorial causation - There can be multiple factors acting independently or cumulatively to produce a disease (non-communicable). As stated above causal factors are not being observed in this study so both causal association and multifactorial causation are ruled out.Common association is not an epidemiological term, it is just a casual term.