The topographic map below has a contour interval of 20 feet. points a and b represent locations on earth’s surface.
which profile best represents the topographic cross section along the line from a to b?

The third option in here is the best option. I you see the interval, you may infer that the cross from point A to point B can not get extremely lower than 500 feet nor extremely higher than 500 feet. specially towards the middle of the way you can see that gets closer to 500 feet and towards the end you can see that amplifies and gets farther than 500 feet. The third options seem suitable 

It would be if we could see the map