**Correct option: (B) 2 cos A**

**Explanation:**

To retrace its path light rays should fall normally on the reflecting surface. So

r_{2} = 0 ⇒ r_{1} = A – r_{2} ⇒ r_{1} = A

Now applying snell rule between incident ray and refracted ray.

(A) sin (2A) = n sin (A) ⇒ 2 sin A cos A = n sin A

⇒ n = 2 cos A