Ans: A
Note that the perpendicular bisector of $OA$ is $y=6$.
Note that the perpendicular bisector of $OA$ is $y=6$.
Since the interaction point of all three perpendicular bisector of $\Delta OAB$ is the circumcentre of $\Delta OAB$. Then the straight line $y=6$ passes through the circumcentre. Hence, the $y$-coordinates of the circumcentre is $6$.
