Scientists have determined that an effective way to lower cholesterol

Study split people with same cholesterol level into two groups: First who ate 3 servings of whole grains, the second did not.
After 6 months, the cholesterol levels of the first group were much lower than those of the second group.

Conclusion: Eating whole grains can have an appreciable effect on cholesterol levels.

To figure out whether it was the whole grains that led to lowering of cholesterol, we need to figure out whether the two groups differed in any other significant aspect e.g. were they following the same exercise regime? were they following the same diet (other than 3 servings of whole grain)? were they taking/not taking the same medicines? etc

Which of the following will be useful to evaluate?

a) Is it realistic to expect people to eat three servings of whole grains per day?

Irrelevant question. First group did eat 3 servings per day.

b) Were the two groups of people in the study involved in the same exercise program?

One of our relevant questions. Did they follow the same exercise program? Say first group exercised for 1 hour 4 days a week while second group exercised for 15 mins everyday. Perhaps the exercise regime of first group was responsible for lowering cholesterol. Answer to this question will be useful to evaluate the conclusion.

c) Can the same drop in cholesterol be achieved through medication?

This question is different from the question above. Had the question been: Were the 2 group following the same medicine regimen?
then it would be relevant. It would tell us a major relevant difference between the two groups.
But as given, if we say "Yes, the same drop can be achieved through medication"
or "No, the same drop cannot be achieved through medication"
Does either impact our conclusion - that the drop was due to 3 servings of whole grains? No.
Even if medicine can give the same drop, it doesn't mean that 3 servings of whole grains cannot. What else can lead to lower cholesterol is irrelevant to us. Whether whole grains led to lower cholesterol or not is out question.

d) Did the study continue to track the subjects beyond six months?

Our study period was 6 months. Whether the study tracked them beyond that is irrelevant.

e) Are most consumers aware of the different between whole grains and processed grains?

"Most consumers" are irrelevant. We know that our subjects of first group ate 3 servings of whole grains and of second group did not.

Answer (B)