ANS:B - a straight line

When we say that the instantaneous centers of three bodies in plane motion lie on a straight line according to Kennedy's theorem, we mean that the points where the bodies seem to rotate instantaneously are all aligned along a single straight line. In practical terms, this implies that if you were to analyze the motions of three bodies moving in a plane, and you were to find the instantaneous centers of rotation for each body at a given instant, these instantaneous centers would all fall along the same straight line. This straight line may not necessarily be horizontal or vertical; it can have any orientation in the plane of motion. The key point is that all three instantaneous centers are collinear—they lie on the same straight line. This observation aids in the analysis of the complex motions of multiple bodies in a plane.