Answer:
A. ∠ACB ≅ ∠ACD.
Step-by-step explanation:
To prove that ∆ABC is congruent to ∆ADC by the SAS postulate:
Side: AC is congruent to AC
Angle: Angle ACB is congruent to angle ACD
Side: BC is congruent to DC
That is, the angle between the two congruent sides must be the same.
Thus, the additional information needed is that ∠ACB ≅ ∠ACD.
A is the correct choice.