Answer
The most appropriate viewing rectangle is $(iii)$. Also see the image below.
Work Step by Step
By simply graphing the function for all of the given viewing rectangles, we can visually determine which of the viewing rectangle is the most appropriate.
The most appropriate viewing rectangle has to include all of the main/critical points and curves of the graph.
As you can see in the image above, the most appropriate viewing rectangle is third $(iii)$ one. Across the borders of the rectangle provided we have not any significance change in the graph.
$(i)$ In the first viewing rectangle it is too much zoomed in. This way we will lose a lot of main points of the graph.
$(ii)$ In this case we will not be able to see the lower parabola-like curve's minimum point. So it is not appropriate.
$(iii)$ This one is appropriate. We have every main point of the graph provided.
$(iv)$ This one is too broad. We won't be ably to see any of the point of the graph properly.