∠ABC = ∠ADC
Here, we want to state an additional piece of information that would make triangle BAC and DAC confgruent by SAS
When we say they both are congruent by SAS, we mean they are congruent by side, angle and side.
From the diagram of the question, we already have two marked sides, with oe on either triangle. We also have the side C being a part of both triangles. So what we need now is the angle that will complete the SAS notation.
The angle in this case is ∠ABC = ∠ADC