D. The supporting details explain that a common assumption about ostriches is not true.
Explanation:
The main idea of a text is the main point the author is making. Supporting details are pieces of information that prove, illustrate, explain, or clarify the main idea.
The given passage tells us about the expression <em>to bury your head in the sand like an ostrich</em>. The main idea is that this expression is based on a wrong assumption - that ostriches bury their heads in the sand when predators are near. The supporting details provide additional explanation. They tell us why this assumption isn't true, giving us the correct information.
