Final answer:
The fallacy shown in the example is Tu Quoque.
Step-by-step explanation:
The fallacy shown in the example of Sarah arguing against adultery while having cheated on her husband is the Tu Quoque fallacy. This fallacy occurs when someone tries to undermine an argument by pointing out the hypocrisy of the person making the argument. However, a person's actions or beliefs have no bearing on the validity of their argument. Therefore, Sarah being a hypocrite does not automatically make her argument wrong.