The correct answer is C) tip leakage loss. In a reaction turbine, the blades are fully immersed in the fluid flow, and the pressure and velocity changes occur gradually along the blade length. This results in tip leakage, where some of the fluid flows from the high-pressure side to the low-pressure side of the blade tips, causing energy losses. In contrast, an impulse turbine has a nozzle that accelerates the fluid before it strikes the turbine blades, and the pressure and velocity changes occur abruptly across the blade. Impulse turbines do not experience significant tip leakage losses. The other options, such as throttling loss (A) and leaving loss (D), are not specific to reaction turbines and can occur in both reaction and impulse turbines. Windage loss (B) is more associated with the rotating components of the turbine rather than the type of turbine.