In a couple, two equal and opposite forces act on a body, but they are not in the same line. The resultant force of a couple is defined as the algebraic sum of the forces, which is zero because the forces are equal in magnitude and opposite in direction. However, the couple produces a turning effect or torque on the body, which is not zero, and depends on the magnitude of the forces and the distance between them. So the answer is (a) zero.