The answer is: " ∠CDA " .
___________________________________________
Note: Complementary angles, by definition, add up to 90°. From the image shown, ∠BDA is a "right triangle:; that is, a "90° angle" . The image shows 2 (TWO) angles that comprise the right triangle, " ∠BDA". Those TWO (2) angles are "∠BDC" and "∠CDA" .
The question asks: "Which angle is complementary to ∠BDC ? From the image shown, the remaining angle that is shown that would complete the right triangle (that is, the 90° triangle—which is ∠BDA) is: "∠CDA" — which is the correct answer.
Note: We know that ∠BDA is a right triangle— that is, a "90° angle"—from looking at that small "square-shaped notation" (in red ink) in the image shown that indicates such.