Official Explanation for Q1
The passage mentions that even if the study had subjects who displayed caffeine sensitivities and a decrease in learning, the positive learning effects from those without caffeine sensitivities would cancel out the decrease in learning from those with caffeine sensitivities. Therefore, if a follow-up study were conducted, in which those with caffeine sensitivities were removed, an even more pronounced effect in caffeine consumption and learning would be evident. This leads us to (D).
(A) is misleading. It almost sounds as though it is referring to the original study. A follow-up study would be more conclusive, at least from the standpoint of showing how pronounced the effect of caffeine on learning is in those without caffeine sensitivities.
(B) is the opposite. A follow-up study would hope to remove any nebulosity or uncertainty from the first study.
(C), like (A) and (B) does the opposite of the follow-up study proposed in the question: it says it would be unhelpful , when in actuality the follow-up study would make the relationship between caffeine and learning clearer.
(D) The answer.
(E) is tempting because it mentions those without caffeine sensitivities. However, the focus is on the observed effect of caffeine on this group and their ability to learn. Differences in long-term memory and short-term memory is a little too specific. It is the fact that the link between overall memory boost and caffeine consumption will become more pronounced if only those without caffeine sensitivities remain in the study.