If your object is moving around the circle at constant speed, then there is some other force acting besides gravity and the normal force of contact with the ring it's traveling around. You can see that from the animations below.
In particular, at your point C there is indeed an inward normal force that will provide the needed centripetal acceleration but there also has to be an upward force as well to keep the object from speeding up as it falls. I'm not sure what the mechanism of that upward force might be, but it has to exist if the object is to move around the circle at constant speed. (Maybe little rocket thrusters?)
I created these animations with a VPython program to show the motion and display the weight as a green vector, the constraint force in red (the normal force is part of this vector), and the net force toward the center of the circle (since it is moving at constant speed). This animation computes the constraint force by simply subtracting the weight vector from the net force.
You can see that for faster motion the constraint force is closer to being normal. It's interesting that the direction changes between the two examples because in the first case the constraint has to "hold up" the object to keep it from falling, and in the second case it has to work more at "holding it in" in a sense.
The third animation shows the components of the constraint vector. The normal force is brownish and the tangential component is a dirty yellow. That tangential component is what is needed to keep the motion going at a steady speed.
