A system of coplanar forces acting on a rigid body can be reduced to A. One force only B. One couple only C. One force and one couple only D. None of the above

The correct answer is: C. One force and one couple only.

A system of coplanar forces acting on a rigid body can be reduced to a single force and a single couple if the following conditions are met:

1. The forces must all lie in the same plane.
2. The lines of action of the forces must all intersect at a single point.
3. The sum of the moments of the forces about any point must be zero.

If these conditions are met, then the system of forces can be replaced by a single force that acts through the point of intersection of the lines of action of the forces, and a single couple that has a moment equal to the sum of the moments of the forces about any point.

Option A is incorrect because it states that the system of forces can be reduced to one force only. This is not always possible, as the conditions listed above must be met.

Option B is incorrect because it states that the system of forces can be reduced to one couple only. This is also not always possible, as the conditions listed above must be met.

Option D is incorrect because it states that none of the above is possible. This is not true, as the system of forces can be reduced to a single force and a single couple if the conditions listed above are met.