The correct answer is B) successive pressure drops through the seal stages. Labyrinth seals used on rotating steam turbine shafts are designed to reduce external leakage by creating a series of small clearances or "teeth" that cause the pressure to drop successively across the seal stages. This stepwise pressure reduction helps to contain the high-pressure steam within the turbine casing, preventing it from escaping to the external environment. The other options are incorrect because: A) pressure increases would allow leakage, C) increased turbulence would not inherently reduce leakage, and D) temperature drops are not the primary mechanism by which labyrinth seals function.