What function is graphed below?

y = x + 4

y = 4x

y = 2x + 4

y = 3x + 2

50%

The very left point on the box of a box plot is the first quartile, the very right. The very left and right points are the minimum and maximum and the middle line is the second quartile.  Since quarters each equal 25%, two quartiles, the data between the first and third quartile, would equal 50%. This is also evident by looking at the graph itself in this problem.