Can someone give me the answer?

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.

Ihave, but this question is gonna probably get deleted.

Ya i could just need to know what type of math