B. It is used to make the principal think about how others might feel about the trees.
In a letter addressing a decision regarding cutting down trees, it is important for the author to appeal to the emotions and values of the reader (in this case, the principal) in order to make a persuasive argument. Stating that cutting down the trees will destroy families' memories is a powerful way to make the principal consider the sentimental value that the trees hold for the community. This can potentially sway the principal's decision and bring attention to the impact of their choice on others.