Answer: SAS
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How to get this answer:
We're told that AD = BC, so that is one pair of sides that are congruent. This forms the first "S" in "SAS"
The "A" refers to the congruent angles, which happen to be angle DAB and angle CBA, both are 90 degrees
The second "S" in "SAS" is the second pair of congruent sides. Those two sides are the overlapping shared side of AB. It might help to peel the two triangles apart to get a better look.
Note how the angles are between the two pairs of sides mentioned.