Where can a couple be placed on a rigid body for an equivalent effect?

A couple can be placed at any point on a rigid body while maintaining the same equivalent effect because a couple is defined as two forces, equal in magnitude, opposite in direction, and separated by a distance. The key characteristic of a couple is that it creates a moment or torque about any point in the body, and this moment will remain the same regardless of where the couple is applied.

When the couple is moved to a different position on the rigid body, it does not change the magnitude or direction of the forces involved; therefore, it still produces the same rotational effect. This principle is crucial in structural analysis and mechanics, as it allows engineers and designers to analyze systems flexibly.

The other options specify limitations on where the couple can be placed, which contradicts the fundamental property of how couples behave in physics. The notion that a couple can only act at the center of mass, at specific supports, or only at the edges does not align with the understanding that the effect of a couple is independent of its location on the rigid body. Thus, stating that a couple can only be effective at certain points is incorrect.