Answer: D. When you cannot find any other sources that support its arguments.
Step-by-step explanation:
A credible source is the one whose information involving the findings and results are unbiased. It is supported by relevant evidences. The information present inside the credible source must be valid, accurate and acceptable by the research society.
A source credibility is also enhanced when you cannot find any contrary evidence against the cited study of the source. As issues may emerge with the content of the study arguing about it's reliability.
Hence, D. When you cannot find any other sources that support its arguments. is the correct option.