Three vectors have equal magnitudes and make 120° with each other. We can say th

assume we have vectors A, B and C. In the question it says the angle between each two is 120 degrees. We can draw vector A on the positive x-axis. Vector B 120 degrees from A which means it is in the 2nd quarter with an angle of 60 degrees above the negative x-axis. Vector C is 120 degrees from A as well. So it is in the 3rd quarter 60 degrees below the negative x-axis. Now, let's examine the components of the 3 vectors:
 Ax = A
 Ay = 0
 Bx = - B cos 60
 By = B sin 60
 Cx = - C cos 60
 Cy = - C sin 60
 since A=B=C
 in the x-axis A - ( 2A cos 60) = 0

Answer should be D