The correct answer is the third option: to show similarities and differences between the Great Wall of China and the Great Firewall. To compare is to examine how things are similar, whereas to contrast is to see how they are different. Consequently, if the author chose to use a compare-and-contrast structure, he did so to look at the similarities of two (or more) objects, in this case the Great Wall of China and the Great Firewall, and their differences.