Final answer:
The last step in the proof that ∆EDG ≅ ∆ADC is justified by the SSS (Side-Side-Side) congruence property.
Step-by-step explanation:
The reason that justifies the last step in a proof that ∆EDG ≅ ∆ADC is SSS (Side-Side-Side) congruence property. This property states that if the corresponding sides of two triangles are congruent, then the triangles themselves are congruent. In this case, if the sides ED, DG, and EG are congruent to the sides AD, DC, and AC respectively, then the triangles ∆EDG and ∆ADC are congruent.