The correct answer is C. Closed polygon.
A force polygon is a diagram that shows the relative magnitudes and directions of a set of forces acting on a body. The forces are represented by arrows, and the sum of the forces is zero if the body is in equilibrium.
A triangle is a closed polygon with three sides and three angles. An open polygon is a polygon that does not close back on itself. A parallelogram is a quadrilateral with opposite sides parallel and opposite angles congruent.
A force polygon representing a set of forces in equilibrium must be a closed polygon. This is because the sum of the forces must be zero, and the only way to represent a zero sum with arrows is to have a closed polygon.
For example, if we have three forces acting on a body, we can represent them as three arrows in a force polygon. The sum of the forces is zero if the three arrows meet at a point. This is only possible if the three arrows form a closed polygon.
If the three arrows did not form a closed polygon, then the sum of the forces would not be zero. This would mean that the body would not be in equilibrium.
Therefore, the force polygon representing a set of forces in equilibrium is a closed polygon.