What is postorder traversal on this simple tree? Given the following tree: 



Which traversal method would give as result the following output: CDBEA?

The answer in my study guide is Postorder, but I think postorder would output: DEBCA. Am I wrong?
You are not wrong. 

A postorder  traversal visits the left subtree, then the right subtree, then the root of the current tree (recursively).