How does the pie chart support the purpose of this
document?

It explains the message in the document.

D. Elizabethans see it as good for nothing but pasture, tin mining, and the steady water supply it provides.

Explanation:

In the excerpt, the author explicitly states that Elizabethans see almost no use in the moor but the ones mentioned in the example, and this is why they prefer not to talk about it. Devonians prefer to highlight some other characteristics about their county instead.