Answer:
b) To prove that artists are not historians
Explanation:
Geoffrey Ashe most likely made the reference in the depiction of biblical scenes because he wanted to prove that artists are not historians.
He made the observation that when you see a painting of an angel appearing to Mary, a window at the back looks like a French chateau which could not have existed in Nazareth.
A chateau is a manor house or where a Lord resides and because a window resembles a chateau in a painting that is supposed to depict the biblical Nazareth, it is likely that the artist that made the painting is a poor student of history.
Geoffrey is of the opinion that artists are not historians (or good students of history) because of the blatant error of putting a French chateau in a painting of Nazareth.