Answer:
A
Explanation:
Consider triangle OAM and OBM. In these triangles,
- OA is congruent to OB as radii of one circle;
- AM is congruent to BM as radii of two congruent circles;
- OM is congruent to OM by reflexive property (this side is common).
Thus, triangles OAM and OBM are congruent by SSS postulate.
Congruent triangles have congruent corresponding parts, so

Hence, OM is angle AOB bisector.