The correct answer is: C. Concurrent coplanar force system.
The force polygon method is a graphical method for solving for the resultant of a system of forces. It is applicable for any concurrent coplanar force system. A concurrent force system is a system of forces that all intersect at a single point. A coplanar force system is a system of forces that all lie in the same plane.
To use the force polygon method, the forces in the system are drawn as vectors, with the tail of each vector at the point of concurrency. The resultant force is then drawn as the closing vector of the polygon. The magnitude and direction of the resultant force can be determined from the length and direction of the closing vector.
The force polygon method is a simple and effective way to solve for the resultant of a system of forces. It is particularly useful for systems of concurrent forces, where the resultant force can be found by simply drawing a polygon of the forces.
The other options are incorrect because they are not applicable to all concurrent coplanar force systems. Option A is incorrect because the force polygon method is not applicable for non-concurrent force systems. Option B is incorrect because the force polygon method is not applicable for systems of non-parallel forces. Option D is incorrect because the force polygon method is not applicable for systems of non-coplanar forces.