Answer: Choice A) Triangle ABC is similar to triangle ACD by AA
AA stands for Angle Angle. Specifically it means we need 2 pairs of congruent angles between the two triangles in order to prove the triangles similar. Your book might write "AA similarity" instead of simply "AA".
For triangles ABC and ACD, we have the first pair of angles being A = A (angle A shows up twice each in the first slot). The second pair of congruent angles would be the right angles for triangle ABC and ACD, which are angles C and D respectively.
We can't use AAS because we don't know any information about the sides of the triangle.
