I believe the answer to this is (B) independent data. Independent data can be obtained in an uncontrolled environment and can also be biased. Scientists depend upon each other to test each others data and results, but this isn't always the case with independent data and therefor the credibility is reduced.
The reason the other answers are incorrect is that unlimited and controlled results do not affect the validity of research, and inconsistent data means just that....that it is inconsistent and most likely requires more research or testing, but isn't necessarily wrong or invalid.
