Question

http://prntscr.com/tkdo6a

Answer 1

The displacement of the body is the line $AB$. Using the cosine law, from the triangle $\triangle AOB$, we can find:

$$AB^2 = AO^2 + BO^2 - 2AB\cdot BO\cos120\degree = 2R^2 - 2R^2\cos120\degree$$

using the trig identity:

$$\cos(180\degree-\alpha) = -\cos\alpha$$

get:

$$AB^2 = 2R^2 + 2R^2\cos60\degree = 2R^2 + 2R^2\cdot \dfrac12 = 3R^2\\ AB = \sqrt{3R^2} = R\sqrt3$$

Answer. C-$R\sqrt3$.