The piece of evidence that adequately supports the assertion that J.R.R. Tolkien "is truly one of the greatest fantasy writers of all time" is:
B: Tolkien has an impressive fan base in America for his two major fantasy novels.
This evidence directly relates to Tolkien's impact and recognition in the realm of fantasy literature, suggesting his works have resonated with a large audience, which is indicative of his status as a great writer in the genre.