D. friendly and respectful because it is a friendly letter, you don't want to be sarcastic which could come off as rude, and you don't want it to be formal either
I would say that it is the job of those who use the internet research ie such as authors to evaluate the credibility of the information gleaned from there and one way is to identify the source ie to verify it say a magazine that is a legitimate entity by calling it or say by asking someone one knows about the validity of the facts one has gathered to confirm them or at least part of them as a sample to test the validity of the whole information. Checking more than one source is a good idea too to corroborate information if much the same answer is obtained from say 2 or more sources it probably has more credibility.
