The answer would be the <u><em>ASA Postulate</em></u> because of the fact that the two opposite angles are congruent and the line crossing the rectangle is congruent to itself (reflexive Property)
The ASA (Angle-Side-Angle) congruence postulate states that if two triangles have two angles and the included side (the side between the vertices of the two angles) congruent, then the triangles are congruent.
The two congruent angles are ∠ACB ≅ ∠CAD and ∠CAB ≅ ∠DCA. The included side is segment AC
