Answer: B) 1/12
Reason:
The horizontal width spans from 8 to 20. This is a distance of 20-8 = 12 units.
The height is some unknown, which I'll call x. We multiply the width and height to get the area. This area must be 1 to represent 100%. You can think of it like a pie chart. This applies to any probability distribution.
area = base*height
1 = 12*x
x = 1/12
Therefore, the height of this rectangle must be 1/12 so that the area of this rectangle is 1.