Two forces acting on an object form a couple. Which statement gives the conditions the two forces must satisfy?

A couple is a pair of forces that produces a turning effect but no resultant force. The forces must therefore be equal in magnitude and opposite in direction so that they cancel as vectors, and they must act along different parallel lines so that the perpendicular distance $d$ between them is not zero. The torque of the couple is $Fd$, so if the two forces shared one line of action $d$ would be zero and there would be no turning effect at all. Unequal forces would leave a resultant force, and forces in the same direction would simply add.