What does Rule 704 prohibit regarding expert witness comments?

The correct answer reflects that Rule 704 specifically prohibits expert witnesses from commenting on whether a defendant had a specific mental state at the time of an offense. This rule is in place because such statements can directly influence the jury's perception and decision-making regarding the defendant's culpability, which is a matter reserved for the jury to determine based on the evidence presented during the trial.

The rationale behind this restriction is that an expert's opinion on a defendant's mental state could cross the line from providing specialized knowledge to effectively offering a legal conclusion, which is the jury's responsibility. By limiting expert testimony in this way, it helps maintain the integrity of the judicial process and prevents potential biases introduced by expert opinions in matters that require a legal determination of intent or state of mind.

In contrast, the other choices address different aspects of expert testimony that are not prohibited in the same way. Experts can certainly comment on the quality of evidence, provide personal opinions about case-related matters as long as they stay within their expertise, and evaluate the credibility of other witnesses, as long as they do so based on factual evidence or their specialized knowledge, rather than making definitive legal conclusions.