Work transfer in any turbine stage comes only from a change in the whirl (tangential) component of the fluid velocity as it crosses the rotor, this is what the Euler turbine equation captures, since a purely axial or radial velocity component carries no torque about the shaft. In Parson's reaction turbine the blades are cambered and the flow direction changes obliquely across both fixed and moving rows, so there is always a strong whirl component present at both inlet and outlet. In a Curtis stage the steam leaves the nozzle with a large whirl velocity and this whirl is stripped away gradually across several rows of moving and fixed blades, so again the whirl component is very much present and non-zero through most of the stage. An impulse turbine with equal blade angles at inlet and outlet still turns the flow through a large angle to generate torque, so the whirl velocities at inlet and outlet remain sizeable even though the blade geometry is symmetric. Only in an axial flow turbine designed for purely axial entry and exit, with the flow travelling parallel to the shaft and no tangential swirl imparted, does the whirl component drop to zero, since there is no tangential velocity component for the blades to act on. This matches option (C).