So we are told that the girls in the class are to be treated as the independent variable. This means if you remember and x and y axis the girls would represent x.
So they are the horziontal axis this eliminates the first and last answer.
From there you can check to see which of the two line up with the graph. Starting from the horizontal axis find 5 and go up vertical until you see a dot on 15 from boys and you can repeat for other numbers as well.
Answer: 2nd graph.