You can easily calculate the using the formula of arc length = (center angle / 360) * 2 * π * r

but, there is a more easy approach. Let's calculate the circumference first = 2 * π * r = 6 * π and the arc length is 4 * π

So, 6 * π will create an angle of 360° with the center cause it's covering the whole circle.

As, 4 * π is 2/3 of 6 * π the certer angle will be = 360° * (2 / 3) = 240°

Answer choice - D