In your response you should note that as the island of Atlantis faces increasing opportunity costs. Starting with no production of potatoes (point F) and increasing production to 200 pounds means decreasing the production of fish from 675 to 650 pounds of fish. Further increase the production of potatoes to 400 pounds means decreasing fish production from 650 pounds to 600 pounds. You will notice that every 200 pounds of potatoes the island must give up more and more pounds of fish. Finally increasing potato production from 800 to 1000 pounds costs 300 pounds of fish. You will notice the production possibilities frontier gets steeper as the island produces more pounds potatoes and few pounds of fish.
The changing opportunity cost implies that the more potatoes Atlantis produces, the higher the opportunity cost becomes. For instance, as you grow more and more potatoes, you have to use less and less suitable land to do so. As a result, you have to divert increasingly more resources away from fishing as you grow more potatoes, meaning that you can produce increasingly less fish. This implies, of course, that the production possibility frontier becomes steeper the farther you move along it to the right; that is, the production possibility frontier is bowed out.