The movement of a body following a circular path is called a circular motion.
1. The total distance measured between the start and ending points is known as displacement.
2. The displacement may be easily estimated once the particle has covered half of the circle by measuring the distance between the beginning and final points.
After half-circle displacement = AB = OA + OB
Therefore, the displacement after completing a half-circle is 2r
So, the correct option is (a)