It depends on what you know or have proven before you get to that statement. Look at the definition of angle bisector and see if what is given and/or what you have proven satisfies all of the required elements of the definition -- then you can say that the definition of angle bisector is the reason. Or see if you have satisfied the requirements of some theorem that you have previously proven that results in a line being identified as an angle bisector -- then give the theorem as a reason.
John
My calculator said it, I believe it, that settles it
