Which additive in hydraulic fluid helps limit oxidation?

In hydraulic fluids, a chemical inhibitor is specifically designed to limit oxidation, which is a process that can degrade the fluid over time and reduce its effectiveness. By adding inhibitors, the hydraulic fluid's lifespan is extended, and the potential for harmful effects from oxidation, such as the formation of sludge or harmful byproducts, is minimized. These inhibitors work by neutralizing free radicals and preventing the reaction with oxygen, thus ensuring that the fluid maintains its performance characteristics for a longer period.

In contrast, the other choices do not serve the same purpose. A corrosion agent would focus on preventing corrosion of metal parts rather than dealing with oxidation, while a lubricating agent is included to reduce friction and wear. A viscosity enhancer modifies the flow characteristics of the fluid but doesn't have any direct role in limiting oxidation. Hence, the additive that specifically targets oxidation is the chemical inhibitor, making it the correct choice.