Because triangle ABC is a regular triangle
So the ACB angle is 60 degrees.
And because BC= 12 OA=8.
So OC=4
So CE=2 (a right triangle with a vertex angle of 30 degrees and a hypotenuse twice as long as a short right side).
So BE=BC-CE= 12-2= 10.
Because of the vertical diameter theorem
So BD=2BE=20.
choose one