The correct answer is A
Explanation
An equilateral triangle has three equal sides, so the lengths of the three arcs must be equal, and thus each arc is 1 over 3 of the circumference of the circle. The circumference of the circle is 2 times pi times r, so the length of arc A X B is 1 over 3 of 2 times pi times r, or (2 times pi times r) over 3.