the figure below shows a rectangle ABCD having diagonals AC and DB.

Answer: b) ΔADC ≡ ΔBCD by SAS
Step-by-step explanation:
Step 1: AD = BC SIDES are congruent
Step 2: ∠ADC ≡ ∠BCD ANGLES are congruent
Step 3: DC = DC SIDES are congruent
The proof is using Side-Angle-Side (SAS)
Look at the angles in step 2. These are the triangles you are proving congruency for: ΔADC ≡ ΔBCD