Which of the following actions is classified as preventative maintenance?

Preventative maintenance refers to routine tasks that are performed to prevent potential issues and maintain the airworthiness of an aircraft. Replenishing hydraulic fluid fits this definition as it involves a simple, routine action performed to ensure that the hydraulic system operates effectively. This maintenance task is often part of regular checks and is intended to prevent larger problems, such as system failure, which can arise from low fluid levels.

In contrast, replacing an engine is a major maintenance action that typically goes beyond the realm of preventative maintenance due to its complexity and the extensive procedures involved. Inspecting wing structure, while essential for safety, is more aligned with the inspection category of maintenance rather than preventative. Performing a 100-hour inspection is a type of detailed inspection mandated by regulations rather than a simple preventative maintenance task. It involves checking various components and systems but does not fall under the routine upkeep that preventative maintenance encompasses.