The correct answer is C. Although Peterson's comments may have been valid at one time, his book is over 30 years old.
This sentence is challenging the credibility of Peterson's book because it is old. At one point, his arguments may have been correct, but this has certainly changed over time given that over 30 years have passed since he published his book. Now, there are newer opinions about the topic, so he isn't a credible source anymore.
In a group discussion, the sentence that is most clearly challenging the credibility of a source is C. Although Peterson's comments may have been valid at one time, his book is over 30 years old. So this means that it used to be credible, but isn't anymore.
