The correct answer is B) floating ring and cylinder body. In a variable displacement radial piston pump, the direction and flow rate of the fluid are determined by the relative positions of the floating ring and the cylinder body. The floating ring controls the displacement of the pump by adjusting the stroke of the pistons, which in turn determines the flow rate. The position of the floating ring relative to the cylinder body dictates the direction and magnitude of the fluid flow. The other options are incorrect because they do not accurately describe the key components that control the fluid flow in a variable displacement radial piston pump. The pump shaft and central valve, the pump shaft and horizontal ports, or the floating ring and pump shaft do not directly determine the direction and flow rate of the fluid in this type of pump.