Looking at the factors above, the common ones are 1, 2, 3, 4, 6, 9, 12, 18, and 36. But... considering we are looking for the "greatest" common factor, our highest number out of the listed ones is 36, making it the greatest common factor of 72 and 36.
Let's factor the top and the bottom. Top factors 4x + 20 = 4(x + 5) Bottom x^2 + 2x - 15 = (x + 5)(x - 3)
The gcf is the largest value that will go into both the top and the bottom. In our problem the ONLY thing that will go into top and bottom is x + 5 so that is your GCF. (c)