NCCER Millwright Practice Exam

The correct answer is "mediums" because in the context of strainers and filters, the term refers to the materials or substances used to trap and remove unwanted particles from hydraulic fluid. These mediums come in various forms, such as mesh, paper, or synthetic materials, depending on the specific application and the size of the particles being filtered out.

Filters function by forcing the hydraulic fluid through these mediums, which captures contaminants while allowing the clean fluid to pass through. This process is crucial for maintaining the efficiency of hydraulic systems and preventing damage to components.

The other options, while related to filtration or fluid dynamics, do not accurately describe the materials used in this specific context. Filters is more a category of devices, sand is typically used in specific types of filtration but not commonly in hydraulic applications, and catalysts refer to substances that increase the rate of a chemical reaction and are not applicable here. Therefore, "mediums" is the most appropriate term to describe the materials employed in strainers and filters for removing particles from hydraulic fluid.