Concept:For three coplanar non-parallel forces in equilibrium, their lines of action must be concurrent (intersect at a single point).
Explanation:When three forces are coplanar, they lie in the same plane.
If they are non-parallel, their lines of action can either intersect at a common point or form a triangle of forces.
For equilibrium, the vector resultant must be zero. This means the three force vectors form a closed triangle when added head-to-tail.
However, for the physical forces to balance without causing rotation, their lines of action must meet at one common point, known as the point of concurrency.
If the lines did not meet at a point, the forces would create a net turning effect or moment, breaking equilibrium.
Being represented in magnitude only or direction only is insufficient — forces must have both magnitude and direction and act through a common point.
Hence, the defining condition is that the lines of action of the three forces meet at a single point.
Answer:D. Their lines of action meet at a point.