The correct answer is: C. concurrent copianar force system.
A force polygon is a graphical representation of a force system. It is constructed by drawing a line for each force in the system, with the length of each line representing the magnitude of the force and the direction of the line representing the direction of the force. The lines are then drawn so that they meet at a single point, called the resultant. The resultant is the single force that would have the same effect on the object as the original force system.
The force polygon method is applicable for a concurrent copianar force system, which is a system of forces that all act at the same point and in the same plane. In this case, the resultant of the force system can be found by drawing a polygon with the lines representing the forces and then finding the closing line. The closing line will represent the resultant force.
The force polygon method is not applicable for a system of parallel forces, because the resultant of a system of parallel forces is not a single force. It is a force couple, which is a pair of equal and opposite forces that act at different points.
The force polygon method is also not applicable for a non-concurrent copianar force system, which is a system of forces that do not all act at the same point. In this case, the resultant of the force system cannot be found by drawing a polygon with the lines representing the forces.