Final answer:
The statement that most weaken a speaker's argument is 'd) The speaker has a personal bias on the topic.' Personal bias can cause selective evidence use and may lead the speaker to present opinions as facts, undermining argument's credibility.
Step-by-step explanation:
The question asks which statement, if true, most weakens the speaker's argument. The correct answer is d) The speaker has a personal bias on the topic.
This is because personal bias can undermine the credibility of the speaker's argument, regardless of their expertise or the strength of their evidence. Bias can lead to a selection of evidence that supports the speaker's viewpoint while dismissing contrary evidence, thereby weakening the argument by not considering all relevant facts and perspectives equally. Depending on what the bias involves, it may also lead the speaker to make inferences that are not fully supported by the data or to present opinions as if they were fact-based inferences, which are not as strong for argumentation as verifiable facts.
Understanding how bias affects an argument is crucial for critical thinking and is also a way to refute the counterclaims by identifying weaknesses in an opponent's stance.