I'm going to use the letters L and W for the length and the width of the granite rectangle. We know that the length is 3 times the width. With this information we can build the following equation:

We also know that the perimeter of the section must be less than 320 inches. The perimeter of a rectangle is giving by two times its length plus two times its width. Then we have the equations:

Since we know that L=3W then:

Now that we know that W=L/3 we can substitute L/3 in place of W on the inequality I wrote before:

This means that the length must be less than 120 inches. This is the same as statement D which is the answer for this problem.