After reading the prompt and the question, you should be looking for things that sever the connection between the premises and the conclustion. For example, an exaplantation for why the people who ate beets had less incidence of cancer that isn't that beets lower the risk of cancer.
A) is irrelevant, because what flax seeds do for cancer has no bearing on beets
B) This actually strengthens the arguement, because it eliminates the possibility that the people in the study generally ate more vegetables and vegetables in general actually decrease the risk of cancer
C) Does nothing to the argument
D) cancer recovery does not equal cancer risk, so this is irrelevant
E) This is the best weakener, it states that those who eat beets are more likey to exercise and leeaves the door open for the arguement that it was the exercise, not the beets that caused the decrease in incidences of cancer