We have here an outline for the proof. Up to step 5, we have used only similarity of triangles and extracted some information on them. Now, we can substitute equation 3 in order to calculate the value of the product of slopes from AC and DC. Hence, the correct choice is where this substitution is shown, hence choice D. We could have trimmed down our choices by noticing that A and C are factually incorrect (-1, not 1)
