Final answer:
In response to a little girl's letter questioning Santa Claus's existence, Francis P. Church's response is best classified as an editorial because it is a piece of writing intended to express an opinion and influence readers.
Step-by-step explanation:
In response to the letter from a little girl questioning Santa Claus's existence, Francis P. Church's writing is best described as an editorial.
An editorial is a piece of writing that expresses the opinions of the author, usually on a topical issue, and is intended to persuade or influence readers. The answer is found by understanding the context and nature of Church's response.
Church's writing does not align with the other options such as a feature article, which typically takes a more neutral stance and provides in-depth reporting on a particular topic.
It is not an essay, which is broader in scope and less about influencing opinion, nor is it an autobiographical sketch, which would be a personal account of Church's own life experiences.
Instead, Church is using the publication as a platform to express a viewpoint on the topic of Santa Claus's existence, making it an editorial.