wondering about the answer here?

@Lucas, The author uses the second paragraph primarily to argue that juries often draw an unwarranted conclusion and provide specific examples when juries are likely to make this type of inferential error, e.g. evidence of prior conviction, photographic evidence, complex evidence. (D) accurately describes the purpose of this paragraph. (B) is incorrect because the author is not saying that evidence must be presented carefully, he merely discusses the most common type of inferential error without any further call for action. (C) is incorrect because it is a variety of an inferential error rather than a commonly held belief that may distort the ability of the jury to ascertain the truth. Let me know if you have any further questions.

It says that the supposition of guilt because of a previous offense is a "commonly held belief". It says that this commonly held belief can lead to errors by the jury. How is this not consistent with answer 3?

That is indeed one way that juries can draw unwarranted conclusions, but it is only one of several issues identified in the paragraph. The main goal is not to explain how multiple common beliefs are problematic, since we only have one instance of a common belief being problematic.