Your hips should be inline with the horse's hips, and your shoulders and hands inline with the horse's shoulders.
Your inside leg (left leg) should be at the girth, and your outside leg (right leg) just a little bit behind the girth. Think of the inside leg as a pole that the horse is bending around. It controls the shoulders so that the horse doesn't become unbalanced and drop his shoulder to the inside. The outside leg controls the hauches. It doesn't allow the haunches to swing out and instead keeps them on the circle.
With your elbows at your side, twist your torso slightly to allow your shoulders to turn with the circle and allow you to see where you are going. This will position your hands so that the inside hand (left hand) is slightly behind the other. If you pull too much with the inside hand, then only the neck will bend and not the spine. When only the neck is bent, it is easy for the horse to evade your aids and not turn at all. Think about steering his withers instead of his nose. Notice how we are not pulling the horse's nose around, instead we are guiding his entire body.
Your seat moves in rhythm with the horses gate, whether at the walk, trot, or canter.
To ask your horse to bend to the right (just like Horse C) switch sides. Your right leg and hand are now the inside aids, and your left leg and hand are the outside aids.
Remember, carrying a rider is not something a horse automatically knows how to do. Whether in the show ring or on the trail, It is the rider's job to help him stay balanced. Once you and your horse are balanced and straight (actually traveling a straight line or an arc) you will be able to ask for more advanced movements and your horse will be prepared to perform them.