A. Corresponding sides of similar triangles are proportional.
1) Note that these triangles Δ ABC and Δ ACD are similar as well as Δ ABC ~Δ CBD in step 3.
2) So we can make use of a property of similar triangles and write down some ratios with them. Similar triangles have proportional sides and congruent angles.
3) Thus, we can state that:
A. Corresponding sides of similar triangles are proportional.