Answer:
A. Definition of angle bisector
Step-by-step explanation:
Given that ΔABC is an isosceles triangle where AB = BC, and that BD bisects ∠ABC, then by the definition of angle bisection of ∠ABC, we have;
m∠ABD = m∠CBD
The correct option is option A. Definition of angle bisector
Also, given that ΔABC is an isosceles triangle and BD is the angle bisector of ∠ABC, we get;
AD = CD and BD = BD
We can therefore, also find that ΔABD ≅ ΔCBD by Side Side Side (SSS) rule of congruency
