Answer:
SAS Theorem
Explanation:
You can tell that the triangles are similar because of the SAS (side-angle-side) theorem.
This is because you already know that two of the sides are congruent since AB and CD were shown to be congruent in the image, and AC and CA are congruent because they're shared. You know that there are similar angles because of the Alternate Interior Angles Theorem*: because of it, you know ∠ACD and ∠CAB are similar because the lines that their transversal intersects are parallel.
*The Alternate Interior Angles Theorem states that if two lines are parallel, the alternate interior angles formed by a transversal are congruent.