The correct answer is B) Reaction. In a reaction steam turbine, the moving blades and the adjacent fixed rows of blades are shaped to act as nozzles. This design allows the steam to expand through the blades, creating a reaction force that propels the turbine rotor. The reaction principle, where the steam pressure and velocity change across the turbine stages, is the defining characteristic of this type of turbine. The other answer choices are incorrect because: A) Impulse turbines use the kinetic energy of the steam to drive the rotor, without significant pressure drop across the blades; C) Helical flow and D) Radial flow refer to the direction of steam flow, not the blade design.