To find the scale factor, we can compare the corresponding side lengths of the two triangles.
The length of the corresponding sides in the two triangles are:
D: 24, 32, 40
D': 6, 8, 10
We can see that each side in D' is 1/4 the length of the corresponding side in D. Therefore, the scale factor used to dilate triangle D to create triangle D' is 1/4