I think that the key point here is the "easily reproducible" - because all of them can be scientific questions (even a! it could tell us about about the human psyche!)
but d. is the best answer for a reproducible experiment. B could also make a good experiment, but it would be too expensive to be easily reproducible. C is calling for a more complex analysis that is too complex for an experiment. And a would require more questionnaires than experiments
