Question

which angle is complementary to ∠adb?

Explanation:

Step1: Recall complementary - angle definition

Two angles are complementary if their sum is 90 degrees. From the figure, we can see that $\angle ADB+\angle BDC = 90^{\circ}$ (as indicated by the right - angle symbol at $\angle ADC$).

Answer:

$\angle BDC$