U2k17-2 Fig. 1 shows an endoscope which consists of an optical tube made of fiber glass with refractive index n. The diameter of the optical tube is d, and it is bent into a semi-circle of inner radius R, to observe the interior of the abdomen through the total internal reflection of light. What is the maximum value of the refractive index n should be?

My answer is A but im not sure
​