Which method would be the BEST way to conduct an observational study to determine the effects of vitamin C for preventing colds?

a) Ask 100 people who consume vitamin C if they got a cold.

b) Divide 100 volunteers into two groups; one group will consume vitamin C and the other group will not consume vitamin C. After a period of time, see how many people in each group developed a cold.

c) Interview 100 people and classify them as either never consuming vitamin C, sometimes consuming vitamin C, or consuming vitamin C on a regular basis. Check for any cold symptoms in all the patients and analyze results.

d) Ask 100 people who had a cold last winter if they consumed vitamin C on a regular basis and record the number.

I initially chose option B, but it was incorrect. Upon further research, I think that option B was more of an experiment. Would option C be the best for an observational study? Options A and D appear to be more surveys, but I am not fully clear on the differences.