I’m not really sure how to do this because I don’t know what options you can select for the answer, but here’s my thinking:
Angle ACB is congruent to angle BCD because of “definition of angle bisector”.
We can also say that angle A is congruent to angle ACB because triangle ABC is an isosceles triangle, and the base angles of an isosceles triangle are the same.
So far, we have angle A is congruent to angle ACB and angle ACB is congruent to angle BCD. We can use the transitive property or substitution (both will work) to get to the final conclusion that angle A is congruent to BCD.
Hope this helped!