When the main reduction gear lube oil system is secured, which of the following components maintains the air within the casing at less than 35 percent relative humidity?

Correct Answer: Option D — Dehumidifier

Dehumidifiers are essential in marine reduction gear systems to prevent condensation and corrosion. By maintaining relative humidity below 35 percent, they protect internal gear surfaces and bearings from moisture-induced degradation during standby periods, ensuring the integrity of the lubrication system and preventing oil contamination while the vessel is at anchor.