Three forces which act on a rigid body to keep it in equilibrium. The forces must be coplanar and A. Concurrent B. Parallel C. Concurrent parallel D. None of these

The correct answer is A. Concurrent.

Three forces which act on a rigid body to keep it in equilibrium must be coplanar and concurrent. This means that they must all lie in the same plane and they must all intersect at the same point.

If the forces are not coplanar, then the body will not be in equilibrium. This is because the forces will be acting in different directions and they will not be able to cancel each other out.

If the forces are not concurrent, then the body will also not be in equilibrium. This is because the forces will be acting at different points on the body and they will not be able to balance each other out.

Therefore, the only way for three forces to act on a rigid body to keep it in equilibrium is if they are coplanar and concurrent.