Where can a couple be moved on a rigid body to maintain its equivalent effect?

The key principle to understand here is that a couple consists of two equal and opposite forces whose lines of action do not coincide, resulting in a rotation about a specific point. One of the defining characteristics of a couple is that it creates a moment (torque) without translating the center of mass of the rigid body.

When considering where a couple can be moved to maintain its equivalent effect, it is important to recognize that as long as the position of the forces that make up the couple is changed in such a way that their effect on the rigid body remains unchanged, it can be moved anywhere. This is because the moment generated by the couple remains the same irrespective of the position of the forces, as long as they remain in a parallel line and their magnitude does not change.

Thus, the couple can be shifted to any point in the rigid body, maintaining the previous rotational effect. This rule applies to any couple configuration, allowing the forces to be relocated without altering the resultant moment acting on the object. This fundamental property is what makes the answer correct.

In terms of the other choices, moving the couple along the line of action or in a parallel plane wouldn't necessarily retain its impact away from the original forces, and moving it along the perpendicular bisector would