Final answer:
Michael Rosen does not provide a reason for the triangular appearance of the letter D, as no specific mention of this is found in the provided texts.
Step-by-step explanation:
According to the author Michael Rosen, there is no specific mention of a potential reason that the letter D used to look like a triangle. While the provided texts explore various historical writing systems, such as Egyptian hieroglyphics, Sumerian scripts, and the evolution of letterforms discussed by Durer, they do not specifically cover Rosen's views on the letter D. However, historical writing systems often evolved from shapes and symbols peculiar to each culture, and these could include triangular forms for various reasons. Nonetheless, without a direct citation from Michael Rosen regarding the triangular shape of the letter D, the correct response would be that Michael Rosen does not provide a reason for the triangular appearance of the letter D.