The correct answer is B. If you discovered that a key informant were a habitual liar.
Step-by-step explanation:
The credibility refers to the quality a source has when all information in it is precise or accurate and therefore can be believed. In terms of news, these are considered as credible if the reporter has been objective, which means he has not manipulated information in the process of reporting it and especially if the source of information is also credible. This implies the circumstance that implies credibility problems is "If you discovered that a key informant were a habitual liar" because this makes the source of information lack of credibility and therefore part or all the information reported can be inaccurate or false.