The correct answer is D. All of the above.
A couple is a system of two equal, opposite, and parallel forces that do not act through the same point. The characteristic of a couple is that the algebraic sum of the forces is zero, but the algebraic sum of the moments of the forces about any point is not zero. This means that a couple cannot be balanced by a single force, but only by another couple with the same magnitude and opposite direction.
A couple can be used to produce a turning effect, or torque, on an object. The magnitude of the torque produced by a couple is equal to the product of the magnitude of the forces and the distance between the lines of action of the forces.
Couples are often used in machines to transmit power. For example, the crankshaft in an internal combustion engine is a couple that transmits power from the pistons to the wheels.