I have to assume that figures OAT and OBT are congruent.
If that is true, the center angle O (of the sector AOB= is 120° and the angle of the sector ACB is 360° - 120° = 240°
Now you can state that the circumferece of the circle / 360 ° = arc ACB / 240° and solve for arc ACB.
arc ACB = circumference * 240 / 360 = 2πr *240/360 = 2π(6)*240/360 = 25.13
Answer: option A. 25.13