The correct answer is the Reflexive Property.
Explanation: The Reflexive Property states that any geometric figure is congruent to itself. In the case of triangles ΔABC and ΔDCB, the side BC is a common side to both triangles. Therefore, we can say that \( BC \cong BC \) due to the Reflexive Property. This congruence helps in establishing the criteria needed to prove that the two triangles are congruent, along with the information Ignacio provided about the congruence of the diagonals.