We are not talking about C as the "end of" bob or bent bar. We are talking about C being the center of mass of the bob or bent bar. It can be either located experimentally(if the object was hanging freely - tied/glued to a string, not like welded to a hinge - C would be directly below the point where the thread is fixed to a ceiling, regardless where on the hanging object itself the thread is tied/glued, when the system was hanging peacefully) or calculated - the second option is much easier if the object is of uniform density and reasonable shape, like a steel orb or a glass. These three points - A, the upper attachment point of the string(to the ceiling), B - the bottom attachment point of the string(to the thing hanging) and C - the hanging object's center of mass, naturally align into one vertical line. The B can actually lie somewhere a bit to the side, if the lower attachment point doesn't rotate freely, but A and C will be in an exact vertical line either way.