The reasoning for the statement "ABD = ACD" is based on the fact that triangles ADB and ADC share a common side AD, and we have already established that BD = DC (since AD is the perpendicular bisector) and that angles ADB and ADC are right angles.
Thus, you can fill in the blank as follows:
Reason: Hypotenuse-Leg (HL) theorem for right triangles.
This theorem states that if two right triangles have a congruent hypotenuse and one corresponding leg that is equal, then the two triangles are congruent. In this case, AD is the hypotenuse and BD = DC is the corresponding leg. Therefore, triangles ADB and ACD are congruent.