This line is describing the hiker's speed.
In the first hour you can see the line is straight, so this means the speed is constant.
From this first hour to the second hour you will see the line drops, and this means the constant speed slows down within that hour.
Finally, from the second hour to the third hour you see a line moving up, and this means the speed increases during this time.
The choice that is correctly represented by this graph would be the third choice.