Final answer:
Triangle ADC is congruent to triangle ADB because reflecting triangle ADC across line segment AD maps it perfectly onto triangle ADB, indicating identical size, shape, and angles.
Step-by-step explanation:
Triangles ADC and ADB are congruent when reflecting triangle ADC across line segment AD maps it onto triangle ADB. The process of reflection across a line segment means that every point of the original triangle is flipped across the line to a point directly on the opposite side, at the same distance from the line. In this case, line segment AD acts as a mirror, and the reflection would produce a triangle identical to ADB, in size, shape, and angle measurements. This transformation positions triangle ADC such that it coincides perfectly with triangle ADB, demonstrating congruence between the two triangles.