What is the reason for statement 5 in this proof?

given: δabc, where ab = bc (view diagram)
prove: m∠bac=m∠bca

statement
reason
1. let δabc be an isosceles triangle with
ab = bc.

given

2. create point d on side ac so bd bisects
∠abc.

constructing an angle bisector

3. m∠abd=m∠cbd

definition of angle bisector

4. bd = bd

reflexive property of equality

5. δabd≅δcbd

6. m∠bac=m∠bca

corresponding angles of congruent triangles have equal measures.

a. asa

b. sss

c. aas

d. sas